ABOUT THIS BOOK
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Boasting significant advantages over liquid finishes, powder coating technology has
received mushrooming acceptance in recent years. It now constitutes 10% of the total
industrial finishing market in North America.
- .._
A major driving force behind the growth of powder coating is environmental regulation of
air, water, and waste disposal. Such regulation has a hard-hitting, detrimental impact on
liquid finishes, which must have venting, filtering, and solvent recovery systems to
control volatile organic compounds (VOCs). Powder, being a dry finish, contains no
VOCs, making powder users much less encumbered by environmental regulations. The
nature of powder coating ais0 makes it economical to use and safer and more comfortable for workers than its liquid counterpart.
Concurrent with powder coating’s growing popularity is research and development
enhancing the technology and opening it up to more applications. These new developments prompted the publication of this third edition of “User’s Guide to Powder Coating.”
The book is a storehouse of updated information for current and potential powder
coating users. Its editor and contributors-people on the cutting edge of the powder
coating technology-provide readers with the most recent hows, whens, and whys of
powder coating.
Darryl L. Ulrich is vice presidenvgeneral manager, Powder Technology Inc., Schofield,
Wisconsin.
As of 1993, Mr. Ulrich spent 34 years in the paint and coatings industry, the last 23
years in powder coatings. His work experience covers the areas of research and
development, sales and technical services, and management in the fields of trade sales
and industrial coati,ngs.
Mr. Ulrich’s powder coating experience began in 1970 at Cargill, Inc. His advent into the
technology was sparked by the development and commercialization of polyester powder
resins for use with urethane curing agents used in some powder coatings. In 1979 he
joined H.B. Fuller Company as laboratory manager of the Industrial Coatings Department. In 1983 he became director of Research and Technical Services of the company’s
Industrial Coatings Division. In 1985 he joined the O’Brien Corporation as technical sales
representative based in Minneapolis, Minnesota. In 1987, Mr. Ulrich became a partner in
a new powder manufacturing company, serving as vice president and general manager
of Powder Technology Inc., a custom manufacturer of powder coatings.
He is a member of the Association for Finishing Processes of the Society of Manufacturing Engineers (AFPISME), and the Chemical Coaters Association. Activities within AFPI
SME include serving as session chair at Finishing ’81 and ‘83 and as conference chair at
Finishing ’85 and ’87. In 1983, he received the AFP Technical Division Contribution
Award.
Mr. Ulrich lectures at the University of Wisconsin Powder Coating Short Course.
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Society of
Manufacturing
Engineers
USER'S GUIDE TO
POWDER COATING
Third Edition
CONTRIBUTORS
Gregory J. Bocchi
Powder Coating Institute
Gerald W. Crum
Nordson Corporation
Charles Danick
Cargill Resin Products Division
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Peter R. Gribble
F e m Corporation
Brad B. Gruss
Fremont Industries, Inc.
Jeffrey W. Hale
Gema-Volstatic Incorporated
c
f
kL
i
I
Steven L. Kiefer
Morton Intemational, Inc.
Ken Kreeger
Nordson Corporation
t
Nicholas P. Liberto, PE
Powder Coating Consultants Division of Ninan, Inc.
i
Gus Lissy
GLA Finishing Systems
i
Kevin Loop
Navistar Corporation
Rick Morgan
Nordson Corporation
Atam P. Sahni
Ferro Corporation
James M. Sales
Ultimate Metal Finishing Corporation
1
George R. Trigg
GRT Engineering
James F. Zeis, CMfgE
Uni Spray
L
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t
USER'S GUIDE TO
POWDER COATING
Third Edition
Darryl L. Ulrich
Editor
Prepared By
Association for Finishing Processes of
the Society of Manufacturing Engineers
SOCIETY OF MANUFACTURING ENGINEERS
Dearborn, Michigan
Copyright 0 1993 by Society of Manufacturing Engineers
Third Edition
98765
All rights resewed, including those of translation. This book, or parts thereof, may not be reproduced
in any form or by any means, including photocopying, recording, or microfilming, or by any information
storage and retrieval system, without permission in writing of the copyright owners.
No liability is assumed by the publisher with respect to the use of information contained herein.
While every precaution has been taken in the preparation of this book, the publisher assumes
no responsibility for errors or omissions. Publication of any data in this book does not constitute
a recommendation of any patent or proprietary right that may be involved or
provide an endorsement of products.
Library of Congress Catalog Card Number: 93-085795
International Standard Book Number: 0-87263-444-2
Additional copies may be obtained by contacting:
Society of Manufacturing Engineers
Customer Service
One SME Drive
Dearbom, Michigan 48121
1-800-733-4763
SME staff who participated in producing this book
Senior Editor: Larry Binstock
Production Secretary: Dorothy Wylo
Association Manager: Cheri Willetts
Printed in the United States of America
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about AFP/SME
The Association for Finishing Processes of SME (AFP/SME) is the principal
educational association for those involved with paint and coating applications
worldwide. It was founded in 1975 to provide an informational interchange among
producers of coatings and pretreatment applicators, product designers, equi ment
manufacturers, users, and coatin s operations professionals, those responsi le for
environmental compliance and inishing consultants.
B
fg
The Association’s goal is to provide leadership and knowledge concerning the
finishing industry. Its objectives are to expand the use of new technologies and
meet the rapid1 changing needs of its constituents. AFP/SME serves individuals
by promoting inishing as a career. It serves companies b promoting technical
excellence. The Association also serves the environment y promoting effective
management and operational techniques.
fy
i:
In pursuit of its goal, AFWSME sponsors conferences, clinics, and courses
around the country. Topics for these sessions include: industrial painting processes, production painting, systems design, troubleshooting coating defects, liquid
and powder coating, radiation curing, plastic finishing, robot finishng, wood finishing, and environmental compliance.
In addition, AFP/SME ublishes books, reports, The Finishing Line technical
quarterly, and pocket gui es to provide current technical information on all aspects
of industrial finishing.
;P
In joining AFP/SME, an individual also becomes a member of the Society of
Manufacturing Engineers and receives Manufacturing Engineering magazine. The
Society presently comprises over 75,000 manufacturing-oriented members in some
70 countries throughout the world.
For more information call the AFP/SME Manager at (3 13) 271-1500, ext. 544.
Preface...................................................................................................................................................... ix
Chapter 1: WHY POWDER COATING?...........................................................................................
1
Economic Considerations................................................................................................................
Applications .....................................................................................................................................
Environmental Implications.............................................................................................................
Powder Coating Growth..................................................................................................................
1
2
2
3
Chapter 2: POWDER COATING MATERIALS..............................................................................
Thermoplastic Powder Coatings......................................................................................................
Thermoset Powder Coatings............................................................................................................
Polyester Resin-based Powder Coatings.........................................................................................
Coating Selection............................................................................................................................
Safety..............................................................................................................................................
5
5
6
9
13
13
Chapter 3: ECONOMIC ADVANTAGES OF POWDER COATING...........................................
17
Cost Savings....................................................................................................................................
Environmental Factors....................................................................................................................
Plant Safety.....................................................................................................................................
Application Evaluation ...................................................................................................................
17
19
19
Chapter 4: SURFACE PREPARATION............................................................................................
25
Cleaning Methods ...........................................................................................................................
Chemical Surface Preparation........................................................................................................
Cleaning Galvanized Steel..............................................................................................................
Cleaning Aluminum........................................................................................................................
Cleaning Steel.................................................................................................................................
25
29
29
31
34
Chapter 5: METHODS OF APPLYING POWDER COATINGS...................................................
39
Fluidized Powder Bed Process ........................................................................................................
Electrostatic Fluidized Bed Process................................................................................................
Electrostatic Powder Spray Coating...............................................................................................
Equipment .......................................................................................................................................
Operation ........................................................................................................................................
Application Factors..........................................................................................................................
Operation Conditions......................................................................................................................
18
39
42
46
47
49
52
53
Chapter 6: POWDER SPRAY BOOTHS AND RECOVERY SYSTEMS.....................................
55
Spray Booths...................................................................................................................................
55
Collector System............................................................................................................................. 59
Color Change Considerations..........................................................................................................
Chapter 7: DESIGN CONSIDERATIONS FOR OPERATION
OF POWDER COATING SYSTEMS.............................................................................
67
Operation of Powder Coating System.............................................................................................
68
Chapter 8: HEATING APPLICATION.............................................................................................
Preheating.......................................................................................................................................
Postheating......................................................................................................................................
Process Considerations...................................................................................................................
Available Methods of Heating........................................................................................................
Oven Cavity Atmosphere...............................................................................................................
Safety..............................................................................................................................................
73
73
73
74
74
75
75
Chapter 9: POWDER STORAGE AND HANDLING......................................................................
77
Storage Recommendations..............................................................................................................
Safety..............................................................................................................................................
78
78
Chapter 10: REPAIR OF PARTS AND HANGER STRIPPING...................................................
Touch-up.........................................................................................................................................
Recoat.............................................................................................................................................
Rebake.............................................................................................................................................
. .
Stnpping..........................................................................................................................................
General............................................................................................................................................
Chapter 11: QUALITY CONTROL...................................................................................................
SPC.................................................................................................................................................
Avoiding and Correcting Quality Variations................................................................................
POWDER COATINGS TODAY AND TOMORROW...................................................................
.
81
81
81
82
82
83
85
85
85
107
Index...................................................................................................................................................... 1 1 1
.
PREFACE
Painting, coating, or finishing-whatever term you choose-has been around for thousands of years.
From the earliest recorded history, and probably long before that, the human race has “painted” for
decoration, protection, and combinations of both.
From the ancient Chinese lacquered or shellacked leather suits of armor to the modern materials of the
space shuttle heat shields, coatings have been used to beautify and protect the substrate or article being
coated.
Coatings have indeed come a long way. However, not all advances were good. Along with improvements in performance, reductions in cost, and increased manufacturing volumes have come many disposal
problems. Toxic wastes entering our air, soil, and water from the finishing industry are being targeted for
drastic reductions and/or elimination. I am not suggesting in the least that the finishing industry is the sole
cause of America’s pollution problems. It is, however, one of the additive factors and does come under
local, state, and federal regulations regarding use and disposal.
The technology of powder coatings offers the finishing industry a way to more easily meet the new
pollution and disposal requirements. Developments in materials and equipment offer the finisher better
quality, wider selection, higher transfer efficiencies, and lower overall finishing costs when compared to
noncompliant coatings.
In the past few years there has been a lot of effort to develop materials that will allow powder into new
markets. Super durable powders are being investigated that will compete in the silicone polyester and
fluorocarbon market. New resin systems will enable formulators to reduce costs in the general purpose
area; perhaps even compete with the “little red wagon paint.’’
Equipment developments allow easier and faster color change or give high enough efficiency so that
reclaim may not be necessary to give the economic advantage to powder.
Suppliers of pigment and other additives recognize the powder industry as a viable market and have
started to increase their efforts to meet industry needs.
New and potential entrants in powder coating manufacturing promise the coater more choices and the
possibilities of small batch, special color runs.
One recent development is a high-temperature silicone powder currently used on muffler and exhaust
manifolds. Operating temperatures and test temperatures exceed 1000°F(538°C). Other lower temperature silicone modified powders are being tested on items such as gas grills. Temperatures are in the range
of 600 to 800°F(316 to 427°C).
ix
Powder is one of the fastest growing new coating technologies. It cannot be used for everything, but
where the substrate is, or can be made, electrically conductive to allow electrostatic spraying and/or
heated to a range of 275 to 400°F(135 to 204"C),powder may well be the best choice. Where they can be
used, powder coatings deliver a combination of quality, economics, efficiency, and safety in the finishing
industry.
To keep the finishing industry abreast of all the many developments in powder coating technology, the
Association of Finishing Processes of the Society of Manufacturing Engineers presents this third edition
of the User's Guide to Powder Coating.
Darryl L. Ulrich
Editor
X
CHAPTER 1
WHY POWDER COATING?
The use of powder coating as a finishing
process has grown significantly in the past several
years. More and more finishing engineers have
turned to powder coating as a way to produce a
high-quality finish while increasing production,
cutting costs, and complying with increasing
environmental pressures. Also, ongoing technological breakthroughs are continually knocking
down the few barriers that hindered powder
coating’s ability to grow in the market. In fact,
powder coating now constitutes 15% of the
finishing market where it competes directly with
traditional liquid finishes. It covers over 10% of
the total industrial finishing market in North
America.
ECONOMIC CONSIDERATIONS
The excellence of the powder-coated finish is
accompanied by substantial cost savings, when
compared to liquid coating systems. Since
powder contains no VOCs, air used to exhaust the
powder spray booth can be recirculated directly to
the plant, eliminating the cost of heating or
cooling the makeup air. Ovens that cure solventbased coatings must heat and exhaust huge
volumes of air to ensure that the solvent fumes do
not reach a potentially explosive level. With no
solvent in powder coating, the exhaust required in
the ovens is lower, resulting in energy and cost
savings in spite of the higher curing temperatures
that powder coating requires.
~
With recent and pending environmental
regulations on air and water emissions and waste
disposal, the switch from liquid to powder
finishing is becoming more prevalent. Liquid
finishing requires the use of solvents to convey
the resinous binder over a surface. These liquid
solvents necessitate venting, filtering, and solvent
recovery systems to control volatile organic
compounds (VOCs). Powder coating, on the other
hand, is a dry finish, which does not use VOCs
during any part of the finishing operation.
In powder coating, the finely ground particles
of pigment and resin are electrostatically charged
and sprayed onto a metal part. The coating
process can be done manually or automatically
with a wide variety of equipment available to
small and large end users. The parts to be coated
are grounded neutral so that the charged particles
projected at them adhere to the parts and are held
there until melted and fused into a smooth coating
in the curing ovens. The result is a uniform,
durable, high-quality finish.
Labor and Efficiency Savings
There are savings in labor costs because less
training is required to operate a powder coating
system, and there is no mixing of powder with
solvents or catalysts. Maintenance costs are also
low, as most clean-ups can be done with a
vacuum.
The powder application system can also bring
greater operating efficiency to the finishing
operation, which can save both time and money.
Parts can be racked closer together on a conveyor,
so more parts can pass through a production line
in a given period of time, resulting in lower unit
costs. More parts can also be coated automatically, as powder coating does not run, drip, or
sag, resulting in significantly lower reject rates.
And with appropriate application equipment,
powder materials, and efficient recovery methods,
one-coat application and overall powder utiliza-
Chapter 1
tion efficiency of 95% to 98% is readily achievable. If more than one color is required, color
change can be accomplished in a relatively short
period of time. And up to 99% of the powder
sprayed at the product surface, but not adhering,
can be recovered and reused, resulting in minimal
waste disposal costs.
powders, over a liquid base coat, are being
developed for exterior auto body finishing.
The architectural and building market uses
powder coating on file cabinets, shelving, aluminum extrusions for window frames, door frames,
and modular office furniture. Posts, rails, fencing, metal gutters, highway and parking lot poles,
guard rails, farm implements, garden tools and
tractors, patio furniture, and other products used
outdoors all benefit from the high weatherability
factor of powder coating.
Today’s powder coatings offer a wide range of
performance properties and glosses, and can
match virtually any color or texture. Film thicknesses from under one mil (.03 mm) to over 15
mil (.38 mm) are possible.
Countless everyday uses for powder coating
include fire extinguishers, mechanical pencils and
pens, thumbtacks, barbecue grills, and vending
machines. Sporting goods equipment uses
include bicycle frames, golf club shafts, ski poles,
and exercise equipment. Technological advancements have allowed expansion of powder coating
to nonmetal surfaces, such as ceramics, wood,
plastic, and brass so that bottles, shower stalls,
dashboards, and even toilet seats are now powder
coated.
APPLICATIONS
Powder coatings are now used in hundreds of
applications. As market potential grows, research
devoted to product improvement also increases,
leading to further innovations and market expansion.
Powder Coating Markets
One of the biggest potential powder coating
users is the appliance industry. The high-quality
finish is both attractive and durable, and a viable
alternative for porcelain enamel and liquid
finishes on traditional appliance surfaces. These
include dryer drums, front and side panels of
ranges and refrigerators, washer tops and lids, air
conditioner cabinets, water heaters, dishwasher
racks, and cavities of microwave ovens. Technological developments have resulted in powder
coatings with lower gloss, lower temperature
curing requirements, and stronger resistance to
chips, scratches, detergents, and grease. All these
features have led to the use of powder coatings on
about 40% of all appliance finishes. Appliance
applications represent about 21% of the North
American powder coating market.
ENVIRONMENTAL IMPLICATIONS
With the current emphasis on control of
emissions from industrial processes and overall
concerns about air quality, ground water, and
hazardous wastes, powder coatings offer an
environmental advantage that may be a determining factor in selecting powder coating as a
finishing process.
No solvents are involved ,in the mixing, application, or clean-up of a powder coating operation,
virtually eliminating solvent emissions and the
need for venting, filtering, or solvent recovery
systems that would be required to control VOCs.
This greatly simplifies the permitting process
needed for installation, expansion, and operation
of facilities, and makes compliance with federal
and state regulations much easier. It also allows
the possibility of including a finishing operation
in a non-attainment area where other systems may
not be permitted.
The automotive industry represents an additional 15% of the North American powder coating
market. Wheels, bumpers, roof racks, door
handles, interior panels, and various “under-thehood” parts are being powder coated. Powder is
also used as a primer-surfacer on component parts
for trucks and recreational vehicles. Clear
2
Why Powder Coating?
Most Powders Nonhazardous
rate. Development work on materials, equipment,
and new applications and surfaces will bring
dynamic changes to the powder coating industry.
Applications not possible a few years ago may
become practical and advantageous in the near
future. The potential powder user should work
closely with suppliers to stay current on the latest
developments in powder coating materials, application, maintenance, and clean-up. And with continually changing environmental regulations, it is best to
check with local officials for disposal recommendations.
In addition, the powders used for powder coatings
are solids and most are classified as nonhazardous.
Their use eliminates or minimizes problems and
expenses associated with the disposal of hazardous
wastes from a finishing process. There is no sludge,
fouled spray booth filters, or solvent to contend with.
Up to 99% of powder overspray can be recovered and
reused, with automatic recycling units collecting the
overspray powder and returning it directly to the feed
hopper, where it returns to the system. In instances
where there is waste, it can be handled as a non-watersoluble solid, presenting few disposal problems.
The use of powder coating as a finishing process
has grown rapidly over the last several years, with
no signs of slowing down anytime soon. This
manual will assist you in understanding the process,
materials, and application methods of this evergrowing finishing technology.
POWDER COATING GROWTH
Because of these and other advantages, powder
coating system installations continue at a dramatic
3
CHAPTER 2
POWDER COATING MATERIALS
Today, powder coatings are available to the
industrial finisher in two major types, thermoplastics and thermosets. Like all other industrial
surface coatings, powder coatings are individually
formulated to meet the industrial user’s very
specific finishing needs. This means matching
the finisher’s house colors and film performance
requirements, all within the particular restrictions
of the finisher’s operation. New types with
unique properties are constantly being developed.
Consequently, as with the selection of any
industrial finish, a close relationship must be
developed between the user of the coating and the
supplier so that these exacting requirements are
thoroughly understood, and the correct type of
finishing material is supplied.
Powder coating materials are unique among all
present-day compliance coatings. They are “dry,”
solid, essentially 100% film-forming coating
materials, as opposed to the liquid materials used
in high solids, water-reducible, radiation cure, and
electrocoat technologies.
Powder coatings are, in fact, not paint until the
coated item emerges from the curing or fusion
oven. Prior to baking, powder coatings are finely
pulverized plastic compositions, and consequently, most resins used in powder coatings are
quite different from those used with liquid paints.
Liquid paints require resins soluble or miscible
with solvents and/or water, whereas powder
coatings require resins that are solid materials.
These solid resins must be solid at ambient, and
reasonably elevated, storage temperatures, and
must be capable of melting sharply to low viscosities to permit flow to a continuous coating film.
when heated.
THERMOPLASTIC POWDER
COATINGS
A thermoplastic powder coating melts and
flows on the application of heat, but continues to
have the same chemical composition when it
solidifies on cooling. Thermoplastic powder
coatings are based on thermoplastic resins of high
molecular weight. The properties of these
coatings depend on the basic properties of the
resin. These tough and resistant resins tend to be
difficult, as well as expensive, to be ground into
the very fine particles necessary for the spray
application and fusing of thin films. Consequently, thermoplastic resin systems are used
more as functional coatings of many mils thickness and are applied mainly by the fluidized bed
application technique.
In the early ’70s the number of solid resin
systems available to powder coatings manufacturers was somewhat restricted and, consequently,
there were limitations on their ability to meet the
diverse needs of the finishing industry. However,
the growth of powder coatings has been accompanied by a rapid technological expansion, resulting
in many new resin systems and other compositional components. This has now made available
a wide range of powder coatings that will meet,
and often exceed, the properties of most presentday solvent-based and water-based baking
enamels.
5
Chapter 2
metal substrates without requiring a primer, and
exhibit good exterior weatherability. They are
good coatings for such items as outdoor metal
furniture.
Typical examples of thermoplastic powder
coatings are:
Polyethylene. Polyethylene powders were the
first thermoplastic powder coatings offered to
industry. Polyethylene provides coatings of
excellent chemical resistance and toughness with
outstanding electrical insulation properties. The
surface of such applied coatings is smooth, warm
to the touch, and of medium gloss. Polyethylene
coatings have good release properties, allowing
viscous sticky materials to be cleaned from their
surfaces. Consequently, they find many uses in
the coating of laboratory equipment.
Thermoplastic powders are most suitable for
coating items requiring a thicker film for extreme
performance. They do not generally compete in
the same markets as liquid paints.
THERMOSET POWDER COATINGS
Thermosetting powder coatings are quite
different from thermoplastic powder coatings;
they are based on lower molecular weight solid
resins. Subjected to elevated temperatures, these
coatings melt, flow, and chemically crosslink
within themselves or with other reactive components to form a higher molecular weight reaction
product. The coating has a different chemical
structure than the basic resin. The newly formed
materials are heat stable and will not soften back
to a liquid on further exposure to heat. Powders
based on these resin systems can be ground to
fine particle sizes, in the range of 25-40 microns.
Due to the rheological characteristics of these
resin systems, they can produce thin paint-like
surface coatings in the 1 to 3 mil (approximately
.025 to .076mm) range with properties equivalent
and sometimes superior to the coatings produced
by the liquid compliance technologies.
Polypropylene. As a surface coating, polypropylene offers many of the useful properties it has
as a plastic material. Because natural polypropylene is so inert, it shows little tendency to adhere
to metal or other substrates. This characteristic
makes it necessary to chemically modify the
natural polypropylene when it is used as a surface
coating powder, so that adhesion of the coating to
the substrate can be obtained.
Nylon. Nylon powders are almost all based on
type 11nylon resin and offer tough coatings that
have excellent abrasion, wear, and impact resistance with a low coefficient of friction when
applied over a suitable primer. A most interesting
use of nylon powder coating is in the field of
mechanical design. Its unique combination of
low coefficient of friction and good lubricity
makes it ideal for sliding and rotating bearing
applications such as automotive spline shafts,
relay plungers, and shift forks, and other bearing
surfaces on appliances, farm equipment, and
textile machinery.
The generic types of resins used in thermosetting powder coatings are:
Epoxy;
Polyester;
Polyvinyl. Polyvinyl chloride powder coatings
have good exterior durability and provide coatings with a medium-soft glossy finish. They bond
well to most metal substrates when applied over a
suitable primer. These coatings will withstand the
stress of metal fabrication operations such as
bending, embossing, and drawing.
New polyester;
Acrylic;
Silicone/Epoxy;
Silicone acrylics; and
Thermoplastic. Thermoplastic polyester
powder coatings have good adhesion to most
Silicone.
6
Powder Coating Materials
Thin-film Epoxy Powder Coatings
In the area of thermosetting powder coatings, a
large technological expansion has occurred in the
last few years. Quite often more than one generic
type of powder coating can be custom formulated
to meet a specific end use. The final choice, of
course, depends on economics and performance,
and must be arrived at through cooperation
between the powder supplier and the user.
In the thin-film surface coating area, Le., in the
1 to 3 mil (approximately .025 to .076mm) range,
epoxy powders are formulated to produce highlyattractive coatings of various gloss or surface
textures, while still retaining the inherent toughness, corrosion resistance, flexibility, and adhesion characteristic of the epoxy resin family.
Recent advances in epoxy crosslinking chemistry
have given formulators a wide range of film
properties within this area. The range of bake
time and temperature has been especially broadened. Epoxies are now available that can be
baked at a temperature as low as 250°F (121°C)
for a period of 20 to 30 minutes, or for a very
short time at much higher temperatures. A good
example of this is the application of epoxy
powder coating to screen wire, where a good
combination of fast cure and thin film forming
ability is required.
Epoxy Resin Based Systems
Epoxy powders are a prominent type of
thermosetting powder coating in use today, due to
their very wide range of formulation latitude.
Epoxy powders are used for both thick-film
functional end uses and thin-film decorative uses.
Crosslinking systems are similar to those used for
epoxy adhesives or two-part epoxy paints, except
most are designed to be stable at room temperatures.
Despite having an excellent combination of
mechanical and resistance properties, epoxy does
have one major disadvantage-poor exterior
weatherability. An epoxy will exhibit rapid
chalking within a few months of exterior exposure, just as a liquid epoxy coating does. Although a weathered epoxy coating may look
unsightly, it still maintains its mechanical and
resistance properties.
Functional Epoxy Powder Coatings
Two major functional end uses for epoxies are
in electrical insulation and corrosion protection.
In the insulation use, epoxy powders conform
exactly to the contours of the electrical part,
bonding to the surface of the part to become a
permanent integral insulation that is void-free and
of low bulk. Typical applications would be on
automobile alternators, electric motors, and
switch gear. In the area of corrosion protection,
epoxy powders provide low-cost, low-maintenance, and long-lasting protection against most
chemically aggressive environments. They are,
therefore, suitable for coating gas and oil field
distribution piping. Recently, multifunctional
epoxies have been used to make high temperature
and pressure-resistant coatings for lining the
inside of “down-hole” oil drilling pipes. Similarly, epoxy powder coated “rebars”-reinforcing
steel bars for highway and bridge decks-drastically cut the formation of corrosion products that
cause concrete cracking and heavy repair costs.
Because of application needs, functional epoxies
are often very fast curing and may require cool
storage and shipment.
Therefore, decorative epoxy powder coatings
are restricted to internal uses or areas of very little
UV exposure. This still leaves a number of
possible application areas for epoxy powder
coatings. Because of their combination of
toughness and resistance properties, epoxy
powders are finding many uses in the major
appliance industry. For example, in some applications white epoxy powder coatings have
replaced porcelain on refrigerator interiors,
providing advantages and cost savings. The
advantages come in the freedom from cracking, a
characteristic displayed by porcelain when
handled roughly.
There is also a cost savings in the type of
substrate that can be used in the liner construe-
7
Chapter 2
tion. Porcelain requires low-carbon steel, which
must be thoroughly pickled through as many as
14 stages. Powder coating can use iron or zinc
phosphated cold-rolled steel. Since porcelain
must be fired at temperatures around 1500°F
(816°C) and thermosetting epoxy powders
normally bake at about 375’F (191°C) temperature range, considerable energy savings are
obtained. Other areas of epoxy powder use in the
appliance industry include the interior coating of
dryer drums and microwave oven cavities.
Similarly, the properties of epoxy powder coatings make them very suitable for automotive parts
such as oil filters, springs, and other components
where exterior durability is not a prime requisite.
Typical applications and general properties of
epoxy are listed in Figures 2-1 and 2-2.
Fire extinguishers
Toys
Refrigerator liners
Dryer drums
Microwave ovens
Mixers and blenders
Fertilizer spreaders
Screening
Oil filters
Automobile springs
Hospital equipment
Bus seat frames
Business machines
Glass bottles
Shelving
Transformer cases
Primers
Bathroom fixtures
Refrigerator racks
Sweepers
Sewing machines
Power Tools
Room air conditioners
Office furniture
Instrument cases
Garden tools
Kitchen furniture
Figure 2-1. Typical applications of epoxy.
that classification misleading. The properties of
these hybrid coatings are more closely akin to
epoxies than polyesters, with a few notable
exceptions. They show similar flexibility in terms
of impact and bend resistance, but produce
slightly softer films. Their corrosion resistance is
comparable to epoxies in many cases, but their
resistance to solvents and alkali is generally
inferior to pure epoxies.
Epoxy Polyester Hybrids
Epoxy powder coatings based on newer
technology are known as epoxy-polyester “hybrids” or “multipolymer” systems.
This group of powder coatings could be
considered simply part of the epoxy family,
except that the high percentage of polyester
utilized (often more than half the resin) makes
One advantage of these hybrids, due to the
influences of the polyester component, is high
ProDertv
Ranae
Hardness (pencil)
Impact resistance (in-lb)
Gloss (60 deg. meter)
Color
Salt spray
Condensing humidity
Cure range (typical 2 mil (0.05”)
film-time @ metal temp.)
HB-7H
60-160
3-100+
All colors, clear and textures
1,OOO+ hr obtainable*
1,OOO+ hr obtainable*
3 min @ 450°F (232%
25 min @ 250’F (121%)
*Environmental resistance properties are a function of the total coating process, including
substrate and pretreatment quality.
Figure 2-2. Typical properties of epoxy.
8
.-
Powder Coating Materials
resistance to overbake yellowing in the cure oven.
This also translates into some improved resistance
to ultraviolet light yellowing. These systems will
begin to chalk almost as fast as an epoxy but,
after initial chalking, the deterioration is slower
and the discoloration less severe than for unmodified epoxy powders.
Toolboxes
Farm equipment
Air conditioner
housings
Electrical control
boxes
Hot water heaters
Hot water radiators
Primer/surfacers
Grain storage bins
Another advantage of epoxy/polyester powder
coatings is their good electrostatic spray characteristics. They can be applied with excellent
transfer efficiency and show good penetration into
corners and recesses.
Oil filters and air
cleaners
Fire extinguishers
Transformer covers
Toys
Screening wire
Power tools
Shelving
Office furniture
Figure 2-3.Applications of an epoxy polyester hybrid.
An epoxy polyester hybrid should certainly be
considered along with the epoxy family for thin
film decorative end use. Applications for an
epoxy polyester hybrid are listed in Figure 2-3;
typical properties are listed in Figure 2-4.
Hydroxyl or Urethane Polyester
Powder Coatings
POLYESTER RESIN-BASED
POWDER COATINGS
The primary type, in use for a number of years,
is a urethane-cured polyester powder that is
comparable chemically to the exterior quality
urethane paints used on aircraft, buses, trucks, and
railroad cars. Coatings of this type combine
outstanding thin film appearance and toughness,
with excellent weathering properties.
The polyester powder coatings used commercially in the United States are divided into two
types (with various curing mechanisms): hydroxyl polyester powders and carboxyl polyester
powders.
ProDertv
Range
Hardness (pencil)
Impact resistance (in-lb)
Gloss (60 deg. meter)
Color
Salt spray
Condensing humidity
Cure range (typical 2 mil (0.05 mm)
film-time @ metal temp.)
HB-3H
60-160
10-loo+
All colors, clear and textures
1,000+ hr obtainable*
1,OW+
hr obtainable*
10 min 0 400°F (204°C)
25 min @ 300°F (149°C)
*Environmental resistance properties are a function of the total coating process, including substrate
and pretreatment quality.
Figure 2.4. Typicalproperties of an epoxy polyester hybrid.
9
Chapter 2
Typical curing times for these types of systems
are 20 minutes at 375°F (191"C) to 10 minutes at
400°F (204°C). This is another area of thermosetting powder coating chemistry that has expanded
greatly in the past few years, and recent developments in the area of cure chemistry have made it
possible to develop good film properties at
temperatures as low as 320°F (160°C) for 20
minutes. However, care should be exercised to
assure that vital properties, such as hardness and
weatherability, are maintained.
Auomotive trim
Parts
Fluorescent light
fixtures
Steel and aluminum
wheels
Patio furniture
Playground equipment
Chrome wheels and
trim
The urethane chemistry of these systems lends
itself to good performance in the thinner 1 to 2
mil (0.25 to .05 mm) film thickness range.
Thicker films tend to exhibit poorer mechanical
properties in such factors as impact resistance and
flexibility. These polyesters are true competitors
to high-quality liquid paints in respect to thin-film
appearance. They have found many uses in the
automotive, appliance, and general metal finishing areas.
Garden tractors
Range side panels and
broiler
Ornamental iron
Air conditioner cabinets
Restaurant furniture
supports
Fence fittings
Transformer cases
Figure 2.-5. Typical applications for urethane polyester
powder.
ness. Other areas of use include office furniture,
wheels, boat trailers, garden tractors, and many
others requiring exterior durability. In the
appliance industry, they are used as a porcelain
replacement on range side panels and meet all the
requirements of this industry's exterior finishing
specifications. Urethane polyester powder
applications are listed in Figure 2-5; typical
properties are given in Figure 2-6.
Many of these systems have an emission
during the cure of 1 to 5% by weight of e-caprolactam, which must be vented from the bake oven
to prevent fouling or appearance contamination.
Carboxyl Polyester Powders
The second type of polyester powder, carboxyl
(COOH) polyester powder, is based on technology developed in Europe. These products can be
described as the exterior durable cousins of the
One major use of these polyesters is in the
coating of lighting fixtures where excellent
reflectivity is required at a minimal film thick-
Property
Hardness (pencil)
Impact resistance (in-lb)
Gloss (60 deg. meter)
Colors
Salt spray
Condensing humidity
Cure range (typical 1-2 mil (0, 5-0.5 mm)
@ metal temp.)
HB-6H
20- 160
5-95+
All colors, clear and textures
1,OW+
hr obtainable*
1,OW+
hr obtainable*
10 min @ 400°F (204°C)
20 min @ 320°F (160°C)
*Environmental resistance properties are a function of the total coating process, including substrate
and pretreatment quality.
Figure 2-6. Typicalpropetties of urethane polyester powder.
10
Powder Coating Materials
epoxy/polyester hybrids but, instead of using a
conventional epoxy resin to co-react with the
polyester, a very low molecular weight glycidyl
or epoxy functional curing agent is used. In this
way, the polyester constitutes a very high percentage of the resin and provides weathering comparable to urethane-cured polyesters.
where a combination of good sharp-edge coverage with good exterior durability is required on
the venting components. Typical applications of
carboxyl polyester powders are listed in Figure 27, and typical properties in Figure 2-8.
These coatings are typically baked for 3 to 15
minutes at 350 to 400°F (177 to 204"C), but more
recent formulations give cures as low as 3WF
(149°C) for extended time periods. This could be
an advantage over the urethane-cure polyesters in
certain applications. The primary attributes of
this type of polyester are their excellent mechanical properties at any film thickness and good edge
coverage. Films of 3 to 5 mils offer excellent
flow, gloss, and toughness. The appearance of
these systems in the 1 to 2 mil (0.25 to 0.05 mm)
range may be a little more textured than the
urethane-cured polyesters but, again, advancement in this area is narrowing the difference.
Overbake color stability is also excellent. Adhesion and corrosion resistance properties are
comparable to the urethane-cured polyesters, but
their resistance to chemicals and solvents is
lower.
Now entering the marketplace are polyester
powder coatings based on new crosslinking
chemistries. They are endowed with excellent
mechanical properties such as high film thickness,
good edge coverage, and dielectric resistance.
Their appearance and properties are similar to
TGIC (carboxyl) polyesters. This type of polyester powder is ideally suited for coating air conditioner cabinets, transformers, and precoated
An ideal application for this type of polyester
powder is the coating of air conditioner cabinets,
Figure 2-7. Typical appications of carboxyl polyester
powders
New Polyester Powders
Irrigation pipe and Fence poles and
fixtures
fittings
Outdoor furniture Lawn and garden
Air conditioning
equipment
units
Aluminum extrusions
Steel and aluminum Transformers
wheels
Wire fencing
Any post-forming
application requiring
weatherability
Prooertv
Range
Hardness (pencil)
Impact (in-lb)
Gloss (60 deg. meter)
Colors
Salt spray
Condensing humidity
Cure range (typical 2-3 mil (0.07-0.5 mm),
time @ metal temp.)
HB-6H
60-160
5-95+
All colors, clear and textures
1,OW+ hr obtainable*
1,OW+ hr obtainable*
3-10 min @ 400°F (204°C)
30 min @ 300°F (149°C)
*Environmental resistance properties are a function of the total coating process, including substrate
and pretreatment quality.
~~
Figure 2-8. Typicalproperties of carboxyl polyester powders
11
Chapter 2
Air conditioning units
Wire fencing
Transformers
Appliances-precoated
makes them excellent candidates for the appliance
industry where they are used quite extensively.
Farm equipment
Aluminum extrusions
The acrylics show good resistance to alkali, an
important trait for use on appliances. On range
sides, for example, they may come into contact
with alkali oven-cleaning compounds, or on
washing machine parts they may be contacted by
hot detergent. These systems also demonstrate
extremely good electrostatic spray properties,
making the application of thin film thickness very
realistic and controllable. As a class, acrylics are
more sensitive to substrate quality than other
thermoset powder coatings, and most acrylics are
not compatible with other coating chemistries.
Typical applications are listed in Figure 2- 11;
properties are given in Figure 2- 12.
metals
Figure 2-9. Tvpical applications of new polyester
powders.
metals. Typical applications are listed in Figure
2-9. Properties are listed in Figure 2-10.
Acrylic Powders
Acrylics constitute another generic group of
thermosetting powder coatings with exterior
durability. In the early days of powder coating,
Propertv
Range
Hardness (pencil)
Impact (in-lb)
Gloss (60")
Color
Salt spray
Condensing humidity
Cure range, time @ metal temp.
F-2H
60- 160
20-90
All colors, clear and textures
1,OW+
hr
hr
1,OW+
10 min @ 400°F (204°C)
30 min @ 375°F (191°C)
Figure 2-10. Typical properties of new polyester powders.
imported acrylics established a good reputation
for exterior durability combined with trouble-free
application characteristics. Major acrylic powder
coatings sold in the U.S. are the urethane-cured
and glycidyl, or epoxy functional, types.
Glycidyl (GMA) acrylics require bake temperatures with a minimum cure temperature
around 300°F (149'C). They have excellent thin
film appearance and offer excellent film flexibility in the form of impact resistance.
Urethane acrylics require bake temperatures
similar to the hydroxyl polyester urethanes with a
minimum cure temperature around 340°F
(17 1"C). They are also characterized by excellent
thin film appearance and, unlike liquid acrylic
paints, retain a level of film flexibility for impact
resistance with excellent film hardness. This
Range side panels
Refrigerator cabinets
and doors
Dishwasher exteriors
Automotive trim
coating
Aluminum extrusions
Microwave ovens
Garden tractors
Washing machine
Parts
Figure 2-1 1. Typical applications of acrylics.
12
Powder Coating Materials
ProDertv
Range
Hardness (pencil)
Impact resistance (in-lb)
Gloss
Color
Salt spray
Condensing humidity
Cure range (typical 1.5 mil (0.0381 mm)
film time @ metal temp.)
2-3H
40- 100
10-90
All colors, clear and textures
1,OOO+ hr*
1,OW+
hr*
10 min @ 400°F (204°C)
25 min @ 350°F(177°C)
*Environmental resistance properties are a function of the total coating process including substrate and
pretreatment quality
.
Figure 2- 12. Tvpical properties of acrylics.
Glycidyl acrylics make the most water-white
clear powder coatings, and are used for various
automotive and hardware applications. Typical
applications are listed in Figure 2-13 and properties in Figure 2-14.
cleanliness, and type of metal pretreatment.
Many specialties in the marketplace can cross
over the guidelines. Because of the mushrooming
effect of this technology, areas of formulation
expertise are developing that can stretch some of
the characteristics of a particular generic type,
making it perhaps a more economically viable
alternate under specific plant circumstances.
Silicone Powders and Modified Silicone Powders
A recently introduced class of silicone powder
coating material can withstand operating temperatures from 400°F (204°C) to over 1,000"F(538°C)
and has found applications in areas from food
contact cookware to engine exhaust systems.
Typical properties of the new silicone powders
are listed in Figure 2-15.
A summary of the key properties of each
generic type of thermosetting powder coating is
presented in Figure 2-16.
In selecting a thermosetting powder type, such
key factors as demonstrated film performance,
demonstrated application characteristics, and cost
performance balance should be kept in focus
COATING SELECTION
SAFETY
Industrial finishes are custom formulated to
individual and end-user requirements. Successful
selection depends on a close working relationship
between users and suppliers. Selection should be
strictly on a demonstrated film performance basis.
This is because the film performance of a thermosetting powder coating is completely dependent
on the bake it receives in a particular plant, on a
particular substrate, with a particular degree of
No matter which coating is selected for a
particular application, the health and safety
aspects of the actual fully formulated and colormatched material to be used must be fully determined. This information should be requested
from the supplier since it can be of great significance to the final application.
13
Chapter 2
Automotive trim coatings
Clear coating for aluminum wheels
Motorcycles
Automotive prime surfaces
Figure 2-13. Tvpical applications of GMA acrylics.
ProDertv
Range
Hardness
Impact resistance
Gloss
Colors
Salt spray
Condensing humidity
Cure range, time @ metal temp.
H-2H
60- 160
10-95+
All colors, clear and textures
l,OOO+ hr
1,OOO+ hr
5 min @ 400°F (204°C)
25 min @ 300°F (149°C)
Figure 2-14. Typicalproperties of GMA acrylics.
Range
Hardness
H-5H
Direct impact
20 in/lbs
Gloss
10-85
Black and other colors depending on system
Colors
15 min @ 440 to 500°F (204 to 260°C)
Cure schedule
400 to >1000"F (204 to >538"C) (Type of substrate is critical)
Operating temperatures
Can be formulated to comply with FDA 175.300 requirements.
Figure 2-15. Typicalproperties of the new silicone powders.
14
Powder Coating Materials
EPOXY
Tough and flexible. Excellent chemical and corrosion
resistance. Excellent mechanical properties. Poor
exterior color/gloss retention.
Epoxy Polyester Hybrids
Decorative film performance. Very good chemical and
corrosion resistance. Very good mechanical properties.
Fair exterior color/gloss retention.
Polyester (Hydroxyl) Urethane
Thin film powder applications. Good chemical and
corrosion resistance. Good mechanical properties. Veq
good exterior color/gloss retention.
TGIC (Carboxyl) Polyester
Thicker film powder applications. Very good chemical
and corrosion resistance. Excellent mechanical properties. Very good exterior color/gloss retention.
New Polyester
Very good chemical and corrosion resistance. Excellent
mechanical properties. Very good exterior color/gloss
retention.
Acrylic Urethane
Thin film powder coatings. Good chemical and corrosion resistance. Poor mechanical properties. Excellent
exterior color/gloss retention.
Acrylic Hybrids
Decorative film performance. Very good chemical and
corrosion resistance. Good mechanical properties. Fair
to good exterior color/gloss retention.
GMA Acrylic
Thin film powder coatings. Good chemical and corrosion resistance. Good mechanical properties. Excellent
exterior color/gloss retention.
Silicone Epoxy
Silicone Acrylic
Silicone
Operating temperatures from 400' to >1000"F(204 to
>538'C). Can be formulated to fit FDA 175.300 applications.
Figure 2-16. Key properties of thermosettingpowder coatings.
15
f
1-
i
i
ECONOMIC ADVANTAGES OF
POWDER COATING
and considerable savings result if it can be
avoided.
Energy and labor cost reduction, high operating
efficiencies, and environmental safety are advantages of powder coating that attract more and
more fmishers. Great cost savings can be found
in each of these areas.
Another significant economic advantage of
powder coating is the minimal amount of oven
ventilation necessary versus the amount needed
for all forms of liquid coatings, including
waterborne, high solids, and electrocoating.
N.F.P.A. 86-A requires that 10,OOO SCF of air be
exhausted from the oven for each gallon of
solvent load; the N.F.P.A. recommends that only
1,500 SCF of air be exhausted for each pound of
volatiles in sprayed powder. Generally, the
amount of volatiles in powder is minimal. Since
this amount will vary from powder to powder, a
user should look critically at this factor in any
potential use analysis.
When compared with a liquid coating system, a
powder coating system has several obvious
significant economic advantages. There are also
many advantages that may not appear significant
by themselves but, when collectively considered,
contribute substantial cost savings. Although this
chapter will try to cover all cost advantages of
powder coating, each individual application must
be analyzed in light of its particular needs, and
appropriate cost advantages must be adapted to
that situation.
COST SAVINGS
Labor Savings
The specific areas covered relative to the
economic advantages of powder coating systems
are: energy savings, labor cost savings, higher
operating efficiencies, environmental factors,
plant safety, and capital costs.
Energy Savings
One of the most significant advantages of
powder coating is that it does not require special
air makeup to the coating booth. Since powder
contains no compounds that are volatile at room
temperature, air makeup for the booth can be
recirculated to the plant-quite advantageous to a
plant where extreme weather conditions are
prevalent. The cost of heating booth makeup air
is a sizeable figure in most coating operations,
>...
17
The labor cost savings of powder coating
depends on the individual finisher’s requirements;
however, there are definite potential labor savings
to be considered. When the powder is delivered
to a user’s plant, it is ready to use; no need to mix
any solvents or catalysts prior to application, as is
necessary for many liquid coatings. Once the
application process is operating, there are no
critical operating parameters to maintain, such as
viscosity and pH, as is the case for many liquid
coatings. Nor are there percent solids, specific
resistance, binder to pigment ratio, and MEQ
levels, which are necessary for electrocoating
systems. The level of skill and training required
of an operator for i powder coating system is less
than that needed for liquid systems, and significantly less than for electrocoat systems.
Chapter 3
There also may be labor savings because better
overall coverage can be obtained with automatic
powder coating equipment. Often less, or no,
manual reinforcement is required. This depends
largely on production requirements and part
configuration, but is a factor to be considered.
abuse better than liquid coatings upon leaving the
oven, less damage will result during the handling,
assembling, and packaging operations. This
decreases the need for touch-up and, again, the
reject rate will be lower. Both items contribute to
savings.
High Operating Efficiencies
Finally, the amount of space required to store
powder, and the space occupied by the powder
coating system itself, is considerably less, in most
cases, than the space required for an equivalent
liquid coating system. This allows more productive and efficient use of available plant space.
Economic advantages resulting from higher
operating efficiencies are many and varied,
depending upon the particular operation. The
most significant advantage is the material usage
efficiency. Fluidized bed operations are inherently 100%efficient, although some loss may
result from such items as dragout and excess film.
Electrostatic spray operations are usually considered to be between 50 and 80% efficient upon
fiist use of the powder. That is, from 20 to 50%
of the material is oversprayed and, if collected,
can be reused as satisfactory powder. Since
oversprayed powder can be reclaimed during the
application process and therefore reused, overall
material utilization in the range of 95 to 98% can
be achieved. By comparison, liquid spray coating
systems can achieve material usage efficiencies
only in the range of 20 to 90%. With
electrocoating, 98 to 99% efficiency is possible.
Capital Costs
Capital costs associated with installing a
powder coating system are becoming more
competitive with a liquid coating system. They
are well below those required for an
electrocoating system. There is also an additional
cost for the laboratory equipment necessary for
maintaining an electrocoat tank. In many cases,
finishers are experiencing a one-year or less
payback period after installing their powder
coating system.
ENVIRONMENTAL FACTORS
Since powder greatly reduces drip, run, or sag,
a significantly lower reject rate can be achieved.
If badly sprayed parts are discovered prior to
curing, they can be simply blown clean with an
air gun, and then recoated. Since there is no
flash-off time required when using powder
coatings, a finisher can use the saved plant space
more efficiently and economically. In addition,
there is less chance of particulate and dust contamination, which could take place during the
flash-off period. This results in fewer rejects for
the powder coating product.
In some cases, it may be more difficult to put a
dollar figure on the economic advantages of
powder coating when considering the environmental factors. However, there are significant
factors that can be readily measured. Since there
are effectively no solvents in powder coating, and
as much as 70% of various solvents in a conventional coating, powder coating can achieve an
environmentally “clean” finishing line.
As regulatory agencies further limit the amount
of solvent emissions allowed, more and more
finishers using liquid coatings systems must
install costly afterburners to incinerate the emitted
solvents. In almost every case, a solvent only
adds to the cost and detracts from the properties
of a cured coating. Another significant environmental factor is the increased difficulty and cost
of disposing of the hazardous waste generated by
liquid coating application. In some cases it is
Powder coating can achieve equal or superior
film properties compared with liquid coatings-in
most cases with only one coat, eliminating the
need to prime a part prior to topcoating.
In addition, since powder coatings develop full
cure during the baking cycle, and usually resist
18
Economic Advantages of Powder Coating
everything that adds cost to the manufacturer
throughout the expected life of the product, must
be considered. This should include items returned
and product liability.
nearly impossible and is a responsibility that
lingers for years.
PLANT SAFETY
These are some of the basic requirements to
fiiish a part:
Consider the economic advantages of powder
coatings in conjunction with plant safety. Since
there are effectively no solvents in a powder
coating, the significant reduction in fire risk could
reduce a plant's h " c e premiums considerably. In addition, any spillage of powder outside
the coating booth can be safely and easily removed by an industrial vacuum cleaner fitted with
an air-driven or dust-tight electric motor. There is
also a reduced health hazard to the operator in a
powder coating system, since there are no solvents to cause nose, mouth, throat, skin, or eye
irritations.
1. Space and equipment for cleaning, pretreat,
application, curing.
2. Manpower.
3. Coating material and supplies.
4. Energy washing, drying, spraybooth and
oven makeup air, curing oven.
5. Waste disposal.
6.Rejected parts due to finish, Le., runs, sags,
scratches, and other damage to the finish.
There are many economic advantages of
powder coating that should be considered when
preparing a justification for a powder system.
The individual requirements and needs for each
application will put a different emphasis on every
considered area, be it energy, labor, operating
efficiency, environment, or safety. It is not
practical in this presentation to cover specific
costs for any single installation. The matter can,
however, be treated in a rather general way. The
following pages include a cost comparison
worksheet and several related tables.
There have been rare cases where a powder
caused skin irritation. Powders can be abrasive
by continued contact with contaminated clothing
or gloves, and proper care should be taken.
Problems can be avoided by washing with soap
and water. The improved worker environment
can result in lower employee absenteeism, which
could be a significant economic advantage.
APPLICATION EVALUATION
Many published articles give cost justifications
for powder with comparisons to liquid. The most
important figure in such evaluations is the net cost
per square foot or per item to successfully coat
the product with a suitable finish.
The examples on the following four pages were
originally developed by Morton Powder Coatings
Group of Reading, Pennsylvania, and have been
adapted for this volume by G. Bruce Bryan of
Volstatic, Incorporated. These cost figures are a
guide, since actual costs change frequently.
Specific costs can be inserted as required.
When considering costs, the entire finishing
system, starting at the entrance area and including
19
Chapter 3
HIGH SOLIDS
LINE #
1
VARIABLE
Paint Cost
L i q u i d $ Per Gal (reduced)
Powder $ Per Pound
2
Volume Solids X
Reduced
3
Specific Gravity
Powder Only
4
Theoretical Coverage
F t ' l t @ 100%
Liquld
Powder
5
LIauI o
EXPLANATION
% Material
2.40
98%
X 1604
866
I Line 3
118
Ut11 i z a t i o n
See Table 3-1 Below
80%
95%
6
F i l m Thlckness M i l s
Average F i l m Thickness
1.2
1.5
7
Actual Coverage
F t ' l l o r Gal.
Line 4
577
75
-0364
,0320
I
8
Applied Cost
SIFt'
PONDER
1.6
192.4
Line 6 X Line 5
~
EXISTING
METHOD
21.00
54%
- Line 2
-
POWDER
EXAMPLE
~
L i n e 1 / Line 7
T A h 3-1
Meterial Utilization
System
X Utilization
Efficiency
Conventlonal
Liauid
Electrostatic
Liauid
Dlsc. o r B e l l
Liauid
Electrostatic
Powder
40
50-70
80-90
95-98
HIGH SOLIDS
LINE #
9
VARIABLE
EXPLANATI O N
LIPUID
Booth Openings Ft'
Square Footage o f 800th
POUDER
EXAMPLE
208
60
125
133
EXISTING
METHOD
POWDER
Openings
10
Face V e l o c i t y Ft/Min
11
CFM Exhausted
Ft3 IMin
12
A i r Temperature
13
14
15
Hininum Avera e V e l o c i t y Is
100 FtlMin, 125 !t/Min
Recomnded
L i n e 9 X Line 10
26,000
Returned
t o Plant
See Table 3-3
Yearly Average Temperature
50 9
50 %
'
Plant A i r
Temperature 'F
Average Plant A i r Temp.
During Winter Months
70 7
70 f
Temperature
Difference "F
Line 12
20 'F
20 or
BTU
BTU
OF
Required
- Llne 13
L l n e 11 X Llne I4 X 1.1'
BTUs/Hour @ 100% Effic.
572,000
0
*The constant 1.1 i s found by m u l t i p l y i n g .018 (BTUs required t o r a i s e one cubic foot o f a i r one degree F. @ 100%
e f f i c i e n c y ) by 60 (converting CFM t o CFH). 1.08 is rounded t o 1.1.
Figum 3-2. Exhaust - spray booth.
20
Economic Advantages of Powder Coating
LINE
16
I
VARIABLE
EXPLANRTION
Production F t
Number o f Parts Per Hour
X P a r t Surface Area
/Hr
/I o r Gal
17
Coverage F t
18
Applied Coating # o r
19
Solvent Load Gal
20
Exhaust Required CFM
Line
POWDER EXISTING
EXAMPLE METHOD
6,000
6,000
577
75
7
/ L i n e 17
10.4
(100% - L i n e 2) X L i n e 18
4.8
1.6
801.6
600
L i n e 16
or f / H r
HIGH SOLIDS
LIOUID
Liquid
Powder
(Mini"
POWDER
80
- L i n e 19
X 167.0(a)
19 X 250.0(b)
- Line
600 CFH)
~-
21
Bake Temperature
22
P l a n t Temp. OF
Line 13
23
Temp. D i f f e r e n c e OF
Line 21
24
BTUs Required
Line 20 X L i n e 23 X 1.1*
OF
Recomaended Cure Temp.
-
325 OF
375 OF
70 '5:
70T
2559
L i n e 22
224.850
305'F
201.300
(a) 167 = 10,000 SCF (NFPA Reconaended) X 1 H r /60 Min L i q u i d
(b) 250 = 1,500 SCF (HFPA Recommended) X 1 H r /60 Min Powder
*
The constant 1.1 f s found by m u l t i p l y i n g .018 B T U ~ requfred t o r a f s e one cubic f o o t o f a i r one degree F @ 100%
e f f i c i e n c y ) by 60 (converting CFM to CFH). 1.08 i s rounded t o 1.1.
Fig-
3-3.
Exh8ust - beke oven.
LINE #
VARIABLE
25
Temp. Difference
26
Conveyor 6 Tooling Load #/Hr
EXPLAATION
"F
L i n e 23
HIGH SOLIOS
LIOUID
POWDER
EXAMPLE
255
305 OF
(Conveyor Wt./Ft. + Hanger
& Hook Wt./Ft.)
X Conveyor
Speed X 60
5,760
5,760
27
Ware Load #/Hr
L i n e 16 X Ware # / F t 2
3.600
3,600
28
T o t a l Load #/Hr
L i n e 26 + L i n e 27
9,360
9,360
29
Specific Heat Load
Use: S t e e l .125, I r o n .13,
Aluminum .248
.I25
,125
30
T o t a l Load Loss BTU/Hr
L i n e 25 X L i n e 28 X L i n e 29
31
Radiation Loss BTU/Hr
T o t a l Oven Surface Area
( F t ) X 3 + X L i n e 25
( I f outslde Use L i n e 12)
1,517,760
1,815,360
L i n e 30 + LIne 31
1.816.110
2,172,210
32
T o t a l Heat Loss BTU/Hr
BTU l o s s per f t * / h o u r w i t h 4" i n s u l a t i o n
F&um 3-4. Conveyor andpsrt heat loss.
21
298,350
356,850
EXISTING
METHOD
POWDER
Chapter 3
LINE #
VARIABLE
HIM SOLIDS
LIOUID
EXPLANATION
POWDER EXISTING
EXAMPLE ETTHOO
33
Exhaust-Spray Booth BTUslHr
L i n e 15
572.000
0
34
Exhaust-Bake Oven BTUs/Hr
L i n e 24
224,850
201.300
35
Conveyor I P a r t Heat Loss
BTUs/Hr
L i n e 32
1.816.010
36
Oven Make-up BTUs/Hr
L i n e 20 X L i n e 14 X 1.1*
37
T o t a l BTUs BTUs/Hr
L i n e 33 + L i n e 34 + L i n e 35
+ L i n e 36
38
39
Energy Cost
Energy Cost
- Gas $/Hr
- Elec.
17,635
( L i n e 37/1,000,000)/Tabl
e
3-2 X Cost/l,W0.000
Cubic Ft.
$/Hr
( L i n e 37/3,415)/Table
X Cost/KUH
2,630.595
19.73
POUDER
2,172.210
13.200
2 n386.710
17.90
3-2
Figure 3-5. Total energy cost.
fable 3-2
Fuel Efficiency
FUEL
Efficiency
LINE t
40
CORL
HEAT
CONTENT X
2.400.000
BTU/Ton 50
VARIABLE
OIL
HEAT
CONTENT X
142.000
BTU/Ton 75
ELECTRICITY
HEAT
CONTENT X
3,415
BTU/KWH 75
EXPLANATION
Labor Costs w/Fringe $/Hr
- Supervision
Pafnt Mix
C - Painters-Liquid(2
0 - Painters-Powder(2
A
B
41
T o t a l Operator Labor f / H r
Total o f Lfne 40
42
Clean-up Cost $/Hr
(Clean-up Man-Hours Per
S h i f t X Labor S/Hr ) /
Operating Hrs /Shf f t
43
44
45
46
47
~
operators)
operators)
A v a i l a b l e Sludge Gal. o r
#/Hr
L i n e 18 X (1.0
X Line 2
Sludge Cost $/Hr
Maintenance Cost $/Hr
TRU/Them 80
HIGH SOLILIS
LIQUID
POWDER
EXAMPLE
15.00
12.00
27.00
15.00
1w,ooo
27.00
1.72
- L i n e 5)
42.00
-60
1.12
3.92
L i n e 43 X Disposal Cost/Gal
or #
4.23
N.A.
(Maint. Hrs I Y r X Maint.
$/Hr.) + Yearly Replacement
Parts Costs + Op. H r s / Y r
5.92
2.53
Line 41 + L i n e 42
L i n e 45 + L i n e 46
Labor IMaintenance $/Hr
GRS
INDIRECT FIRE
HEAT
CONTENT X
54.00
( F i l t e r Cost X I o f F i l t e r s
Per Year) t Operating Hrs / Y r
F i l t e r Cost $/Hr
GRS
DIRECT FIRE
HEAT
CONTENT f
100~000
BTU/Therin 90
~~
Figure 3-6. Labor and maintenance costs.
22
+ L i n e 44
.96
N.A.
t
66.83
45.13
EXISTING
HETHOD
POWDER
Economic Advantages of Powder Coating
HIGH SOLIDS
LIQUID
VARIABLE
Material Costs
Energy Costs
- Line 8 X
- Line 38 X
Line 16 X Operating HnNr
$425,880
$374.400
$38,474
$34,905
$130.319
$88,004
$594,673
5497,309
5.0508
f.0425
operating H n / Y r
- Line 47
Labor & Maintenance Costs
X OperatingHnNr
Total Operating Costs
POWDER
EXAMPLE
Annualized Cost/Ft *
Total Operating Costs / (Line 16 X Operating HnNr)
EXISTING
METHOD
Figure 3-7. Cost summary: Annualized dollars.
Depreciation
Energy Credits
Tax Credits
Reject/Rework Costs
Safety Costs
Health Consideration Costs
Figure 3-8. Other costs to consider.
t i b h 3-3
Average Temperature
(Canadian cities are in degrees Celsius)
LOCAT I ON
Albany, N Y
Albuquerque, NH
Atlanta, G4
Baltimore, MD
Bismarck, NO
Boston, HR
Buffalo, NY
Burl ington, VT
Calgary, Alta.
Cheyenne, WY
Chicago, IL
Cleveland, OH
Columbia, SC
Concord, NH
Dallas, TX
Denver, CO
Des Woines, IA
Detroit, MI
Great Falls, MT
Hartford, CT
Honolulu, HI
Houston,
TX
Indianapolls, IN
Jacksonvllle, FL
Juneau, A K
Kansas City, MO
L i t t l e Rock, AR
Los Angeles, CA
Louisville, KY
*
Source:
LOCATION
AVG.*
4a
55
62
56
41
50
47
45
4
45
50
49
64
44
66
50
50
49
46
50
75
69
53
69
42
55
62
63
57
Memphis, TN
Miami, FL
Milwaukee, WI
Minneapolis, HN
Mobile, AL
Montreal, Que.
Nashville, TN
New Orleans, LA
New York, NY
Oklahoma City, OK
Omaha, NE
Philadelphia, PA
Phoenix. AZ
Portland, ME
Providence, RI
Quebec, Que.
Rapid City, SO
Reno, N V
Richmond, YA
S t . Louis, MO
S a l t Lake City, UT
San Francisco, CA
S e a t t l e , WA
Toronto, Ont.
Vancouver, B.C.
Washington, DC
Wichita, KS
Winnipeg, Man.
Weather Bureau/Department o f Comnerce
23
AVG.*
62
75
48
45
68
7
60
70
52
60
51
54
70
46
50
5
47
50
58
50
51
56
52
8
11
56
57
3
POWDER
c
CHAPTER 4
SURFACE PREPARATION
one truly stands alone-cleaning. Cleaning is the
single most important function in maximizing
powder coating results. The degree of cleaning or
surface modification is open to a wide interpretation and many differing qualities. For some
applications, abrasive cleaning alone is suitable,
especially if the final product requires short-tomedium duration adhesion properties.
The highest quality powder coating will
provide excellent results only if the pretreatment
(cleaning, phosphatizing, conversion coating) is
done correctly and the overall pretreatment
system is maintained up to its potential. Powder
coating brings pretreatment back to the basics of
cleaning, rinsing, phosphatizing, and seal rinsing.
To maximize the benefit derived from powder,
the pretreatment system must provide a clean
conversion coated product in a dry state to the
powder booth.
Leading powder coating and pretreatment
manufacturers tend to agree that a high-quality
five-stage spray system of cleaning, iron phosphatizing, and final deionized rinsing is the most
suitable and preferred method of pretreatment for
the widest range of substrates and coating chemistries. Work closely with your pretreatment and
powder coating vendor, and select those who
work well together on your behalf.
Surface preparation encompasses more than
conditioning or improving a substrate to accept
the powder coating. Items that should also be
considered include:
Soils on the substrate;
Water quality for bath makeup and rinsing;
CLEANING METHODS
Consistent or varying substrates;
Metals mix-ferrous,
metals;
Mechanical methods of cleaning can be
generally applied to all types of surfaces since
they involve abrasion of a surface by scrubbing,
sanding, grinding, tumbling or vibrating with
abrasive media, and wet (slurry) or dry abrasive
blasting. Along with the cleansing action (removal of loose soils and tightly adhering contaminants such as mill scale on steel), these processes
may be selectively performed in a manner that
produces an etched or "toothed" surface to
improve the adhesion of coating films to the
substrate. Prominent among the methods used to
achieve both purposes simultaneously is abrasive
blasting. Methods and systems of surface preparation by dry blast cleaning include:
nonferrous, yellow
Metal types-cast, extruded plate, coil, sheet,
or combinations;
Size, weight, dimensions, and configurations
of product; and
System control, maintenance, and record
keeping.
Considering all key elements involved in
producing a high-quality powder-coated product,
25
Chapter 4
enclosures, automated blast nozzle systems, and
other innovations in the airblast systems have
contributed in large measure to achieving environmentally acceptable airblast processes.
Open airblast with sand or various slag-type
abrasives.
Manual airblast in environmentally controlled
enclosures (airblast rooms), with or without
materials conveyor systems.
Airless (Centrifugal Wheel) Blast
Automated centrifugal wheel blast.
0
Introduced in the 1930s was a device that
hurled abrasive material by centrifugal force.
Machine systems available for various applications differ only in the means by which the
product is conveyed through the blast, the number
and size blast wheels required, and the type of
blast media used. All centrifugal blast systems,
whether for shop installation or for portable use,
have the same six basic components:
Centrifugal wheel and airblast systems, in
tandem or at separate locations in the path of
product flow.
Airblast
The practice of blasting parts for cleaning or
improving material surfaces goes back many
years. It was called “sandblasting” because it was
accomplished by introducing ordinary sand into a
column of high velocity air supplied by an air
compressor, and manually directing the stream of
sand at the surface to be treated. However,
because a wide variety of abrasives are commonly
used, the process is more appropriately called
“airblasting” or “nozzle blasting.”
1. A blast enclosure is provided to contain the
abrasive as it is thrown from the wheel and
to prevent generated dust from escaping to
surrounding areas.
2. As part of the system, there is a means of
presenting the workpiece to the blast.
3. The heart of the system resides in the wheel
or wheels, in whatever size and number
required for a specific application.
Portable nozzle blast systems have been
developed in which an abrasive, after it has
impinged the surface, is entrapped at the blast
head, recovered by vacuum, and recirculated.
The result is a virtually dust-free blasting operation in the field.
4. A means of recapturing and recirculating the
spent abrasive is normal in all industrial
systems.
These developments have extended many
benefits in varying degree, according to the nature
of the blasting operation. Operating costs have
been reduced through re-use of abrasives, automated processing, and/or improved working
conditions for blast operators. Improved environmental conditions and reduced air pollution have
resulted from the use of portable, recoverable
abrasive blast systems and blast enclosures. The
capacity for processing greater numbers and types
of parts, including very large parts, in a shop
operation has been increased.
5. To remove contaminant particles and
abrasive too small to be effective, an
airwash separator is included, which then
returns the cleaned and usable abrasive to a
storage hopper.
6 . Essential is a dust collector that withdraws
dust from the abrasive and ventilates and
removes dust from the blast enclosure.
For applications in which the type and shape of
parts and the process requirements may be
accommodated for centrifugal blasting, the
economy benefit has been proven many times
over. Production rates are greatly increased, and
Blasting with nozzles is a practical production
process, used in several industry classifications.
In many situations, it is the only means of achieving the preparation process. Modern airblast
26
Surface Preparation
production costs significantly reduced. Airless
blast cleaning operations are far less laborintensive and far more energy-efficient than
airblasting. Uniformity of quality in the finished
product is enhanced in automatic and environmentally clean operations.
Removing dry soils, mold, sand, and paint;
Removing burrs, scratches, and surface
irregularities;
Providing an anchor pattern for better adhesion of paints or other coatings;
Method Selection
Producing a hammered or matte surface
finish; and
Either method of abrasive blasting may be
incorporated readily into continuous in-line
conveyorized production cleaning and coating
operations. Generally, the airless method is more
productive and economical than airblast. In
airblast systems, accelerating the abrasive particles (particularly the higher density ferrous
abrasives) requires high volumes of clean, dry,
compressed air, which in turn demands high
energy input. Centrifugal blast systems require
only about 10%of the energy (horsepower)
needed by airblast systems to throw equal volumes of abrasives at the same velocities. Conversely, the shape (or configuration) of the
workpieces may dictate the selection of airblast
systems because of the ability to position individual nozzles to direct the blasts into nooks and
crannies that may be shadowed from the blast
pattern of the centrifugal wheel. Often, the two
methods may be used in tandem.
Etching and carving for decorative finishes.
Types of materials that are abrasive blasted to
prepare the surface for subsequent finishing
operations and/or to produce the desired final
surface finish include:
Ferrous and nonferrous castings, forgings,
rolled and formed shapes, and weldments;
Thermoplastic and thermosetting plastic
parts; molded rubber parts; and
Diverse types of materials, including glass,
wood, and leather.
Abrasives
Numerous types of materials can perform
surface preparation by abrasive blasting. Selecting blast media depends on the type of material
and product to be processed and on the nature of
the surface finish to be produced. A wide range
of abrasives is available.
Successful abrasive blast cleaning and/or
finishing operations require judicious selection of
the ,combination of blast system and abrasive
media to achieve the desired finish on the material
and configuration of the part to be processed.
Applications
Naturally occurring abrasives, generally
referred to as “sands,” include silica sands as well
as heavy mineral sands (with little or no free
silica) such as magnetite, staurolite, and olivine
rutile. Others used for specialty-type operations
include garnet, zircon, and novaculite. Generally
speaking, the use of these materials is limited to
nozzle-blast systems.
Abrasive blasting involves high-velocity
propulsion and direction of abrasive particles
against surfaces of parts or products, for removal
of contaminants and/or otherwise conditioning the
surfaces for subsequent final finishing operations.
Typical uses of abrasive blasting include:
Mineral slags, derived from metal smelting and
electric power generating slags (bottom-ash), are
a rapidly growing source of abrasives for cleaning
new, corroded, or painted steel surfaces. As with
Removing contaminated surface layers;
Removing oxides, corrosion products, and
mill scale;
27
Chapter 4
Parts on which oil or grease are present cannot
be blast-cleaned properly. These substances resist
removal and, in recirculating abrasive systems,
may contaminate the abrasive itself. It is, therefore, essential that oil and grease deposits be
removed by solvent-wiping or degreasing methods prior to blast cleaning.
sand, the use of these highly aggressive products
is generally limited to the airblast systems.
Byproduct abrasives are mainly agricultural in
character. Byproducts such as walnut and pecan
shells and peach pits are excellent for use in both
air and airless blast systems for removing paint,
fine scale, and other surface contaminants without
altering the metal substrate. Corncob particles are
effective for removing surface contaminants such
as grease and dirt without destroying or altering a
painted or bare metal surface. Media of these
kinds are also frequently used in deflashing
molded rubber and plastic parts.
Surfaces to be blast cleaned should also be dry,
although traces of moisture can be tolerated when
the heat generated by the blast is sufficient to
vaporize moisture. Larger amounts of moisture
cause the dust fines generated during the blasting
to adhere to the part surface. Failure to remove
such dust may cause defects in a coating system.
Excessive moisture also causes dust to adhere to
the abrasive particles and, in a recirculating
abrasive system, will inhibit thorough air-wash
cleaning of the abrasive and the removal of dust
fines from the system.
Manufactured abrasives may be nonmetallic.
Included in this category are glass beads for
peening and cleaning small, delicate parts, and
plastic media commonly used (like the agricultural by-products) for deflashing molded rubber
and plastic parts. Materials such as aluminum
oxide and silicon carbide are frequently used for
specialty etching or for cleaning metals such as
stainless steel, aluminum, and copper. Aluminum
oxide is frequently used (primarily in airblast
systems) for cleaning and etching steel.
Blast profile should be consistent. Situations
where media is allowed to degrade, or where
control is lacking, cause incomplete scale and
corrosion removal. This produces a non-uniform
substrate and a subsequent decrease in ultimate
powder coating durability. Where there is a
combination of non-uniform profile and low-mil
build of powder, reduced corrosion protection is
common. This is caused by the peaks of the
profile extending above or through the powder
coating.
The most commonly used manufactured
abrasives are cut stainless steel wire and aluminum shot, principally for surface preparation and
shot peening of nonferrous alloy parts.
Some manufactured abrasives are ferrous
metal. Three general types are available for
surface preparation-chilled cast iron shot and
grit, malleable iron shot and grit, and cast steel
shot and grit. Of these, the cast steel abrasives are,
by far, the most frequently used. Since they are
available in broad ranges of size and hardness,
these abrasives have applications ranging from
deflashing rubber parts with shot to etching
hardened steels with hardened grit.
Advantages
Abrasive blasting has numerous advantages,
especially when combined with subsequent
aqueous conversion coatings or phosphatizing.
The prepared substrate is void of scale, corrosion,
and soils; it is uniform, and offers mechanical
adhesion properties along with under-film corrosion protection similar to that of chemical conversion coatings.
Precautions
Mechanical blasting offers advantages over
chemical removal when cleaning hot rolled steel
substrates. Mechanical is faster, safer for workers
and, when controlled, provides a more uniform
profile that is less likely to re-oxidize. Abrasive
Soils that adhere to the blasted surface or to the
abrasive particles may be the source of defects in
a subsequently applied coating, such as paint or
enamel.
28
Surface Preparation
or glass bead mechanical conditioning of nonferrous castings of zinc and aluminum is in many
cases the easiest way to deal with problematic
castings for powder coating.
cleaned are hung in the solution and are the
anode, while other electrodes act as the cathode.
Electrocleaning is more effective than plain
immersion due to the scrubbing action of the gas
bubbles produced at the surface of the part.
CHEMICAL SURFACE
PREPARATION
The hand-wiping method of application derives
additional benefit from the physical act of removing the soil from the surface by means of a cloth
or sponge, with the cleaner helping to solubilize
the soils.
The chemical surface preparation used in any
particular application is closely related to the
nature of the surface being cleaned and the nature
of the contamination. Most surfaces powder
coated after cleaning are either galvanized steel,
steel, or aluminum. Since not all chemical-type
preparations are applicable to all these materials,
the preparation process selected depends on the
substrate material. For each material, the type of
cleaning will be discussed and its unique features
for that substrate will be explained. Specific
application processes are quite similar for each
material.
Alkaline cleaners are usually applied to galvanized zinc surfaces in two stages-the cleaning
stage and a water rinse stage. The parts to be
cleaned are usually conveyed from one stage to
the other after suitable exposure to produce
cleaning. Additional stages of cleaning and
rinsing may be used if required. The chemicals in
the baths for this type are usually maintained at a
temperature between 80 and 200°F (27 and 93°C).
Typically the temperature is 120 to 150°F(49to
66°C) for spray and 150°F(66°C) for immersion.
The time for which the parts are exposed to these
chemicals is between 30 seconds and 5+ minutes.
Generally, it is 1 to 2 minutes for spray and 2 to 5
minutes for immersion. To be effective, the
concentration of such alkaline cleaning solutions
should be between 1/4 and 16 odgal(2 to 120 g/
L). Typically, in the spray the concentration is 1/2
to 1 odgal(4 to 8 g/L) and for immersion 6 to 12
odgal(45 to 90 g/L).
CLEANING GALVANIZED STEEL
Alkaline Cleaners
Alkaline cleaners for galvanized steel usually
have a blend of mild alkaline salts that do not
damage the zinc surface. In some cases, a smallto-moderate amount of free caustic soda may be
present in the cleaner to remove difficult soils or
to provide a desired etch. These cleaners can be
applied by power spray, immersion,
electrocleaning, or hand wipe.
The most expensive of these types is the
electrocleaner, due to the higher bath concentrations used and the cost of electricity for the
electrocleaner. The least expensive is the spray
cleaner, with hand-wiping somewhere in between.
The alkaline type is, by far, the most effective and
usually the least expensive to operate. In order of
decreasing performance, the methods of application would generally be rated as: electrocleaning,
spray cleaning, immersion cleaning, and handwiping.
In the power spray method, the parts to be
cleaned are suspended in a tunnel while the
cleaning solution is pumped from a holding tank
and sprayed, under pressure, onto the parts. The
cleaning solution then is continuously recirculated. Spray pressure ranges from 4 to 40 psi.
In the immersion method, parts to be cleaned
are simply immersed in a solution of the cleaner
contained in a mild steel or stainless steel tank.
Acid Cleaners
Electrocleaning is a specialized version of
immersion cleaning in which a direct current is
passed through the solution. The parts to be
Acid cleaners are not normally used to clean
galvanized steel. Of those acid cleaners that are
29
Chapter 4
In order of decreasing performance, the
methods of application would generally be rated
as: electrocleaning, spray cleaning, immersion
cleaning, and hand-wiping.
used, the most common type would be mildly
acidic salts, not too corrosive to the zinc surface.
It should be noted, however, that there are specialty acid cleaners designed to remove white
corrosion product from galvanized surfaces.
Neutral Cleaners
In the power spray method of application, the
parts to be cleaned are suspended in a tunnel
while the cleaning solution is pumped from a
holding tank and sprayed under pressure onto the
parts. The cleaning solution then is drained back
into the holding tank and the cycle repeated. The
pumping, spraying, and draining operations take
place simultaneously and continuously.
A neutral cleaner (as used for galvanized steel)
may be composed of surfactants only, neutral
salts plus surfactants, or surfactants with other
organic additives. A neutral cleaner may be
defined as any cleaner which, in solution, would
register between 6 and 8 on the pH scale.
When the immersion method of application is
used, the parts to be cleaned are simply immersed
in a solution of the cleaner contained in a mild
steel or stainless steel tank.
With the power spray method, parts to be
cleaned are suspended in a tunnel while the
cleaning solution is pumped out of a holding tank
and sprayed, under pressure, onto the parts. The
cleaning solution is continuously recirculated.
Spray pressure ranges from 4 to 40 psi.
Electrocleaning with acid cleaners is a specialized version of immersion cleaning in which a
direct current is passed through the solution. The
parts to be cleaned are usually the anode, while
other electrodes act as the cathode.
Electrocleaning usually produces a cleaner
surface than plain immersion due to the scrubbing
action of the oxygen bubbles coming off at the
surface of the part. The oxygen is a result of the
electrolysis of the water.
The hand-wiping method derives additional
benefit from the mechanical aid of physically
moving the soil from the surface by means of a
cloth or sponge with the cleaner helping to
solubilize the soils.
~
-
In the immersion method of application, the
parts to be cleaned are simply immersed in a
solution of the cleaner contained in a mild steel or
stainless steel tank.
Once again, hand-wiping has an additional
benefit from the mechanical aid of physically
moving the soil from the surface by means of a
cloth or sponge, with the cleaner helping to
solubilize the soils.
Neutral cleaners are usually applied by using a
minimum of two stages: the cleaning stage and a
water rinse. Additional stages, a cleaning and
rinsing, may be used if required. The solutions are
held at a temperature of 80 to 200°F (26 to 93°C);
typically 120 to 160°F(49 to 71°C) for spray and
150 to 180°F(66 to 82°C) for immersion. The
parts are exposed for 30 seconds to 5+ minutes;
typically 1 to 2 minutes for spray and 2 to 5
minutes for immersion. The solutions are held at a
concentration of 1/4 to 16 odgal(2 to 120 gL);
typically 1 to 2 odgal(8 to 16 g L ) for spray and
8 to 14 odgal(60 to 105 g/L) for immersion.
Acid cleaners are usually applied to galvanized
zinc surfaces in two stages: the cleaning stage and
a water rinse. Additional stages, cleaning and
rinsing, may be used if required. The chemicals in
the bath are maintained at a temperature of 80 to
200°F (27 to 93°C); typically 100 to 140°F(38 to
60°C) for spray and 140 to 180°F(60 to 82°C) for
immersion. The parts are exposed to the chemicals for 30 seconds to 5+ minutes; typically 1 to 2
minutes for spray and 2 to 5 minutes for immersion. The solutions are kept at a concentration of
1/4 to 16 odgal(2 to 120 gL); typically 1/2 to 1
odgal(4 to 8 g L ) for spray and 4 to 12 odgal(30
to 90 g/L)for immersion.
Neutral cleaners are not effective as the
primary cleaner. They are more likely to be used
as a precleaner.
30
Surface Preparation
the solution is pumped from a holding tank and
sprayed under pressure onto the parts. The coating
solution is continuously recirculated.
Conversion Coating of Galvanized
Steel
Iron phosphates or cleaner-coater products
produce little or non-detectable conversion
coatings on zinc surfaces. Many multimetal
finishing lines use modified iron phosphates
which offer cleaning, and leave micro-chemical
etch on zinc substrates to provide adhesion
properties.
In the immersion method of application, the
parts to be coated after being cleaned are simply
immersed in a solution of the phosphating solution contained in a stainless steel tank.
The hand-wiping method of application has
limited use in conversion coating technology.
Many municipalities and states now have limits
on zinc PPMs, forcing metal finishers to provide
treatment of any solutions in which zinc substrates are processed.
Phosphate coatings are usually applied by
using five, six, or seven stages. The phosphate
solution is held within a temperature range of 100
to 160°F(38 to 71°C) for spray; 120 to 200°F (49
to 93°C) for immersion; or room temperature for
hand-wiping.
The zinc phosphate conversion coating is,
perhaps, the highest quality coating that can be
produced on a galvanized surface. To produce a
zinc phosphate coating on galvanized, special
accelerating agents are required to activate the
surface sufficiently to receive the zinc phosphate
coating. These coatings are created by the action
of bath chemicals on the surface materials. A
crystalline zinc phosphate is actually “grown” on
the clean substrate surface. In a typical sevenstage zinc phosphating unit, the various stages
are:
The applied zinc phosphate coating weight
should be 150 to 300 mg./sq. ft.
A processing time of 30 to 60 seconds by spray
and 1 to 5 minutes by immersion is usual.
Phosphating solutions have a concentration of 4
to 6% by volume and are applied at spray pressure of 5 to 10 psi.
The zinc phosphate coating is probably one of
the best paint base coatings on galvanized steel.
A chromium phosphate processing solution does
not produce a suitable paint base coating on
galvanized steel.
1. Alkaline cleaner.
2. Alkaline cleaner.
3. Hot water rinse.
CLEANING ALUMINUM
4. Zinc phosphate processing solution.
Alkaline Cleaners
5. Cold water rinse.
Alkaline cleaners for aluminum differ from
those used for steel; they usually have a blend of
mild alkaline salts to avoid attacking the aluminum surface. In some cases, a small to moderate
amount of free caustic soda may be present in the
cleaner to remove difficult soils, or to provide a
desired etch.
6. Post treatment (either chromium or
nonchromium type).
7.Deionized water rinse.
A six-stage unit would eliminate stage 1, and a
five-stage unit would eliminate stages 1 and 7.
In the power spray method of application, the
parts to be cleaned are suspended in a tunnel
while the cleaning solution is pumped from a
In the power spray method of application, the
parts to be coated are suspended in a tunnel while
31
Chapter 4
between. The alkaline type is, by far, the most
effective of the cleaner types and usually the least
expensive to operate.
holding tank and sprayed, under pressure, onto
the parts. The cleaning solution is continuously
recirculated. Spray pressure ranges from 4 to 40
psi.
In order of decreasing performance, the
methods of application would generally be rated
as: electrocleaning, spray cleaning, immersion
cleaning, and hand-wiping.
In the immersion method of application, the
parts to be cleaned are simply immersed in a
solution of the cleaner contained in a mild steel or
stainless steel tank.
Acid Cleaners
Electrocleaning is a specialized version of
immersion cleaning in which a direct current is
passed through the solution. The parts to be
cleaned are usually the anode, while other electrodes hanging in the tank act as the cathode.
Electrocleaning is more effective in cleaning than
plain immersion due to the scrubbing action of the
oxygen bubbles coming off at the surface of the
part. Oxygen results from the electrolysis of the
water.
Acid cleaners for aluminum are composed of
mildly acidic salts or a phosphoric acid base. In
either case, any oxide film on the aluminum will
be removed by the acidic medium. The acid
cleaners are usually not as effective in cleaning
common soils as the alkaline cleaners.
In the power spray method of application, the
parts to be cleaned are suspended in a tunnel
while the cleaning solution is pumped from a
holding tank and sprayed, under pressure, onto
the parts. The cleaning solution is continuously
recirculated.
The hand-wiping method of application derives
additional benefit from the physical act of removing the soil from the surface by means of a cloth
or sponge, with the cleaner helping to solubilize
the soils.
When the immersion method of application is
used, the parts to be cleaned are simply immersed
in a solution of the cleaner contained in a mild
steel or stainless steel tank.
Alkaline cleaners are usually applied to aluminum by using a minimum of two stages: the
cleaning stage and a water rinse. Additional
stages, a cleaning and rinsing, may be used if
required. The chemical baths are held at a temperature of from 80 to 200°F (27 to 93"C),
typically 100 to 140°F (38 to 60°C) for spray and
140 to 180°F(60 to 82°C) for immersion. Parts
are exposed to the chemicals for 30 seconds to 5+
minutes; typically 1 to 2 minutes for spray, and 2
to 5 minutes for immersion. A bath concentration
of 1/4 to 16 odgal(2 to 120 g/L)is used; typically 1/2 to 1 odgal(4 to 8 g/L)for spray and 6 to
12 odgal(45 to 90 g/L) for immersion.
Hand-wiping derives additional benefit from
the physical aid of removing the soil from the
surface by means of a cloth or sponge, with the
cleaner helping to solubilize the soils.
Acid cleaners are usually applied to aluminum
using a minimum of two stages, the cleaning
stage and a water rinse. Additional stages, a
cleaning and rinsing, may be used if required. The
acid solutions are held at a temperature of 80 to
200°F (27 to 93°C); typically 100 to 140°F(38 to
60°C) for spray and 140 to 180°F(60 to 82°C) for
immersion. Parts are exposed for 30 seconds to
5+ minutes; typically 1 to 2 minutes for spray and
2 to 5 minutes for immersion. Solutions are held
at a concentration of 1/4 to 16 odgal(2 to 120 g/
L) for spray and 6 to 12 odgal(45 to 90 g/L) for
immersion.
Comparing the cost of using the various types
of chemical cleaners, the most expensive would
be the immersion electrocleaner due to the higher
concentrations used and the cost of electricity for
the electrocleaner.
The least expensive would be the spray
cleaner, with hand-wiping being somewhere
32
Surface Preparation
Conversion Coating of Aluminum
Comparing the cost of using various cleaners,
most expensive would be immersion due to the
higher concentrations used. The least expensive
would be the spray cleaners, with hand-wiping
somewhere between.
A standard iron phosphate processing solution
can only produce a coating on aluminum if certain
accelerators are used. The coating produced,
however, is not an iron phosphate, but an aluminum phosphate. The accelerator normally used is
a proprietary product. In phosphating an aluminum surface, a chemical action takes place on the
surface, producing a crystalline coating on the
surface. Iron phosphating of aluminum is not an
ideal operation but is usually expedient where
mixed product-iron and aluminum-is being
coated on a single line.
In order of decreasing performance, the
methods of application would generally be rated
as: spray cleaning, immersion cleaning, handwiping.
Neutral Cleaners
A neutral cleaner for aluminum may be composed of surfactants only, neutral salts plus
surfactants, or surfactants with other organic
additives. A solution of a neutral cleaner will
usually register between 6 and 8 on a pH scale.
In the power spray method of application, parts
to be coated are suspended in a tunnel while the
coating solution is pumped from a holding tank
and sprayed, under pressure, onto the parts. The
coating solution is continuously recirculated. One
specialized spray application uses a steam generator, where the chemical is siphoned into the steam
at the nozzle. Spray pressure ranges from 4 to 40
psi.
In the power spray application, parts to be
cleaned are suspended in a tunnel while the
cleaning solution is pumped from a holding tank
and sprayed under pressure onto the parts. The
cleaning solution is continuously recirculated.
Spray pressure ranges from 4 to 40 psi.
In the immersion method of application, the
parts to be coated are simply immersed in a
solution of the coating solution contained in a
mild steel or, preferably, a stainless steel tank.
The hand-wiping method of application derives
additional benefit from the physical act of removing the soil from the surface by means of a cloth
or sponge with the cleaner helping to solubilize
the soils.
The hand-wiping method of application has
limited use in conversion coating technology.
Neutral cleaners usually are applied to aluminum using a minimum of two stages: the cleaning
stage and a water rinse. Additional stages, a
cleaning and rinsing, may be used if required.
Neutral cleaners are held in the temperature range
of 80 to 200°F (27 to 93°C); typically 120 to
160°F (49 to 71°C) for spray and 150 to 180°F (66
to 82°C) for immersion. Parts are exposed to the
cleaners for 30 seconds to 5+ minutes; typically 1
to 2 minutes for spray and 2 to 5 minutes for
immersion. The chemical concentration is between 1/4 to 16 odgal(2 to 120 g/L)
typically l
to 2 odgal(8 to 15 g/L) for spray and 8 to 14 o d
gal (60 to 105 g/L) for immersion.
A minimum of three application stages should
be used for adequate quality; five for best quality.
The solutions normally are held at a temperature
between 98 and 120°F(32 and 49°C) for spray
and 130 to 160°F(54 to 71°C) for immersion.
Coating weights of 5 to 40 mglsquare foot are
normal. Parts are exposed to the chemicals for 1
to 2 minutes by spray and 3 to 5 minutes by
immersion. Concentrations of 10 to 15 lbs/100 gal
by spray and 20 to 30 lbs/1000 gal by immersion
are applied at a spray pressure of 5 to 10 psi.
The aluminum phosphate coating produced
from an iron phosphating solution is a relatively
good paint base. The process is easy to operate
and maintain, and can be applied in as few as
three stages. The process, however, cannot be
Neutral cleaners are not effective as the
primary cleaner. They are more likely used as a
precleaner.
33
Chapter 4
used exclusively for aluminum as this will shorten
the effective life of the bath. To maintain the
proper balance of chemicals in the bath, some
steel must also be processed. The balance of
chemicals in the bath also can be maintained
through the use of special proprietary additives.
phate coating stage in the processing line at least
once a year.
Chromium Phosphate
The chromium phosphate conversion coating is
probably the best conversion coating paint base
that can be applied to an aluminum surface. It is,
however, limited to treating aluminum.
Zinc Phosphate
A zinc phosphate processing solution will
produce a very good quality coating on aluminum. It is one of the best paint bases produced on
an aluminum substrate.
When power spraying is the method of application, the parts to be coated are suspended in a
tunnel, while the coating solution is pumped from
a holding tank and sprayed, under pressure, onto
the parts. The coating solution is continuously
recirculated.
In the power spray method, the parts to be
coated are suspended in a tunnel, while the
coating solution is pumped from a holding tank
and sprayed under pressure onto the parts. The
coating solution is continuously recirculated. A
specialized spray application uses a steam generator where the chemical is siphoned into the steam
at the nozzle. When immersion is the method of
application, the parts to be coated are simply
immersed in a solution of the coating solution
contained in a mild steel or, preferably, stainless
steel tank.
In the immersion method of application, the
parts to be coated are simply immersed in a
solution of the coating chemical contained in a
stainless steel tank.
The hand-wiping method has limited use in
corrosion coating technology.
When aluminum parts are chromium
phosphated, usually five stages are used. The
solutions are held at a suitable temperature
between 70 and 130°F(21 and 54°C). A coating
weight of 20 to 100 mghquare foot is normal.
Parts are exposed to solutions with concentrations
of 2 to 5% by volume for a time of 5 to 60
seconds. Solutions are sprayed at a pressure of 5
to 10 psi.
The hand-wiping method has limited use in
conversion coating technology.
To adequately create a zinc phosphate coating
on aluminum, five operation stages are usually
required. The temperature of the solution is
between 108 and 160°F(38 and 71°C) for spray
and 120 to 200°F (49 to 93°C) for immersion.
Coating weights of 50 to 200 mg/square feet are
usual. A time of 1 to 3 minutes by spray and 2 to
5 minutes by immersion is needed. Solutions
having a concentration of 4 to 6% by volume are
applied at spray pressure of 5 to 10 psi.
The chromium phosphate conversion coating is
superior to zinc phosphate as paint base on
aluminum. It exhibits better salt spray resistance
and offers improved adhesion. It is less expensive
to maintain and is easier to control. However, it is
limited to use on aluminum only and requires a
stainless steel tank.
A zinc phosphate processing solution can be
used exclusively for producing a conversion
coating on aluminum. It is a very good paint base,
but produces a lot of insoluble sludge. This sludge
can deposit on plate coils, causing a decrease in
heat transfer efficiency. It can also plug up the
nozzles and piping in a spray application. It is,
therefore, necessary to clean out the zinc phos-
CLEANING STEEL
Cold-Rolled Steel
Commonly referred to as “mild steel” or CRS,
cold-rolled steel is available in coil, sheet, and a
34
Surface Preparation
variety of forms. The surface is free from heat
scale and, if properly protected by a corrosion
preventive or adequate packaging, is free of rust.
HRS,hot-rolled steel, is also available in coil,
sheet, and a variety of forms. The surface is
typically covered with a bluehlack heat scale,
which will flake off upon bending or flexing the
metal. HRPO,hot-rolled, pickled, and oiled refers
to steel that has been pickled to remove the blue
heat scale and oiled to prevent rusting.
Solutions of alkaline cleaners can be applied
to a soiled surface by mixing the aqueous solution
with steam and spraying the stedcleaner
mixture on the soiled surface through hand-held
steam guns. When the elevated temperature is not
required, diluted aqueous alkaline solutions can
be pressure-sprayed on the soiled surface. Rinsing
is usually accomplished without the addition of
cleaner solution. The surface is either blown dry
with compressed air or allowed to air dry. When
sprayed, the cleaner solution is continuously
applied to the soiled surface for a sufficient time
to permit acceptable cleaning and is then rinsed in
the same manner. The temperature of the cleaner
solution is nominally controlled by the quantity
and temperature of the steam mixed with the
cleaner solution. Application using a hand-held
gun by one person should be made only to an area
where adequate cleaning and rinsing, with a
minimum of streaking and flash drying, is obtained. Nominal concentration of the solutions
requires 1% to 1 odgal at the nozzle.
During the manufacturing and/or assembly of a
particular piece of ware, the steel surface can
become soiled with drawing compounds, a variety
of lubricants or lubricating oils, shop dirt, weld
splatter, marking inks, chalk, etc. These must be
removed for a good adhering finish. Knowing the
chemical nature of these soils will help in their
removal.
Alkaline Cleaners
Because steel surfaces are resistant to attack by
aqueous alkaline solution, an extremely wide
variety of alkaline cleaners can be formulated for
each specific application. In general, combinations of phosphates, silicates, and carbonates,
with varying amounts of caustic, may be encountered. In addition, sequestering agents or chelating
agents, solvent, solvent couplers, dispersants, and
one or more surfactants, nonionic or anionic, are
used.
When alkaline cleaners are used with the
immersion process, the surface to be cleaned is
immersed or dipped into a cleaner solution
contained in a tank. Added features of heat,
agitation (mechanically or by means of ultrasonic
cavitation), or the application of electric current to
permit electrocleaning may be incorporated. The
ware is immersed in the cleaner solution until
acceptable cleaning is obtained. Simple immersion cleaning followed by ultrasonic cleaning or
electrocleaning is only used where cleaning
requirements are very severe. Solution temperatures are usually between 140°F(60°C) and a
gently rolling boil. Typical immersion times are
on the order of 2 to 5 minutes. Extended times, up
to one hour, may be used where heavy or difficult-to-remove soils are encountered. The concentrations of the solutions for light duty are 4 to 8
odgal(30 to 60 gL), and for heavy duty 6 to 16
odgal(45 to 120 gL).
Liquid, moderately alkaline cleaner solutions
may be applied to the surface to be cleaned, using
a hand-held rag, brush, mop, etc. The “wetted”
soil is allowed to soak for a period of time and is
then spray rinsed with water and/or dried by ragwiping or blowing the surface dry with compressed air. The cleaner solution can be applied
and reapplied until a sufficiently clean surface is
obtained, which is then rinsed. Applied with a
hand-held rag or brush, the solution is normally
unheated or warmed only slightly. Application
should be made only to an area that can be
handled by one person, to provide cleaning and
rinsing with a minimum of streaking, drying, etc.
When the solutions are to be wiped or brushed
onto the surface, concentrations of 5 to 20% are
used.
Where a power spray washer is used, the ware
to be cleaned or treated is suspended in a housing
with appropriate pumps and piping, so that a
heated solution is pressure-sprayed on the soiled
35
Chapter 4
market viscous, brush-on rust and soil removers.
Typically, acid rust removers are not used through
a steam gun. A great amount of derusting of steel
parts is accomplished by pickling, in hydrochloric
or sulfuric acid, ware which has previously been
alkaline cleaned. Strong acid cleaners are typically not used in spray equipment, except in
highly specialized operations, such as pickling in
primary steel mills. Moderate or mild acid
cleaners are used in spray applications in limited
cases or where direct-fired gas heating is encountered.
surface. The solution flows on to the ware, runs
off, and is diverted into a holding tank, from
which it is continuously recirculated through a
pump back onto the ware. Spray pressure must be
sufficient to provide coverage of the ware with
solution, and not excessive enough to cause wild
movement of the hanging parts. Nominally, spray
pressures are in the 20 to 40 psi range, more
typically 15 to 20 psi.
A cleaner stage followed by a rinse stage is
used for most of the work processed today. Where
heavily soiled surfaces are encountered or a
higher degree of cleanliness is required, additional stages are used. Depending upon the
demands of the operation, cleaning is done in:
Iron Phosphate
A number of different proprietary compositions
can be used to produce an iron phosphate conversion coating on steel surfaces. These compositions are all mildly acid and are principally
monosodium phosphate in either liquid or powdered form, to which are added various accelerating agents and other ingredients. When the
functions of both cleaning and coating are desired
in one stage, a detergent surfactant package can
be incorporated with the iron phosphate composition in one package, or can be added separately at
tankside, when required.
2 stages; clean/rinse;
3 stages; clean/clean/rinse; or
4 stages; clean/rinse/clean/rinse; or
clean/clean/rinse/rinse.
Typical hot-process spray cleaners operate in
the 140to 160°F(60to 71°C)range. Lowtemperature cleaners nominally operate in the 90
to 120°F (32to 49°C)range. When parts are
carried by a monorail conveyor, cleaner stages of
45 to 60 seconds, and rinse stages of 30 to 45
seconds are typical. Solution concentrations used
where the chemicals are powders are 1/2 to 2 o d
gal (4to 15 g/L), and where they are liquid 1 to
Moderate or strong acid proprietary compositions, capable of removing rust and containing
surfactants and solvent to remove oils and soils,
can be wiped or brushed on steel to produce a
light iron phosphate coating. Aqueous solutions
are applied using a rag or mop. The solution can
be reapplied where and when necessary, until the
surface is clean and free of rust, and the light iron
phosphate has developed. The time required is
determined by what is needed for the manual
application of the solution to a suitable area, to
remove the soils and produce the coating. The
solution, applied by hand-held rag or brush, is
normally unheated. Liquid acid products are used
at 5 to 10% for light cleaning and 10 to 25% for
moderate cleaning, rust removal, and the deposition of the iron phosphate coating.
3%.
Acid Cleaners
Mineral acids, i.e., sulfuric or hydrochloric
acid, are commonly used to remove rust, heat
scale, and corrosion products from steel. Organic
acids and phosphoric acid, together with solvents,
coupling solvent, and surfactants are used to both
clean (remove soils) and remove red rust and
other corrosion products.
Liquid acid rust removers containing detergents can be applied by hand, provided impervious protective gloves and other suitable protective
equipment is used. There are also available in the
Steam gun phosphatizing is best suited for
extremely soiled substrates where the steam and
chemical melt and displace tenacious wax-laden
36
Surface Preparation
Depending upon the soil conditions of the ware
and the quality level requirements, spray systems
may consist of as few as two stages (cleaner/
coater in one, water rinse in two), and as many as
nine stages. Such washers are referred to as:
solids or preservatives. Although steam-gun
cleaning and phosphatizing works well, the
process is typically manual and the effective
cleaning area is only as wide as each pass of the
width of the steam gun.
Hot-water high-pressure cleaning and phosphatizing hand-held systems are gaining popularity with custom coaters and manufacturers of
large substrates or products. This application
equipment generally operates at a minimum of
800 to 1600 psi, and at from 3 to 5 gpm of
solution. Temperature ranges from a low of 140
to 200°F (60 to 93°C) at the nozzle tip, with
chemical concentrations of from 1 to 3%by
volume. Iron phosphate coating weights can
reach 35 to 45 mgft2, meeting TT-C-49OC Type I1
requirements. When considering the use of high
pressure, hot water, hand-held systems it is
important to have systems built of stainless steel
components as the phosphatizing solutions are
acidic. Recent advances in this area also incorporate wet sandblasting attachments, which are
effective in thoroughly cleaning weldments.
Two-stage: cleadcoat, rinse;
Three-stage: cleadcoat, rinse, post rinse;
Four-stage: cleadcoat, rinse, post rinse, D.I.
flush;
Five-stage: clean, rinse, treat, rinse, post
rinse;
Six-stage: clean, rinse, treat, rinse, post rinse,
D.I. flush;
Seven-stage: clean, clean, rinse, treat, rinse,
post rinse, D.I.;
Eight-stage: clean, rinse, clean, rinse, treat,
rinse, post rinse, D.I.; and
Iron phosphate conversion coatings can be
applied by immersion application methods.
Where light soils are encountered, the cleaning
and coating can be accomplished in one stage,
preferably with the assistance of some form of
agitation. In general, immersion iron phosphate
coatings will tend to look streaky and be of a light
gray-blue color. Where soils are light, parts can
be cleaned and coated in one stage, followed by
water rinsing. Moderate to heavy soils require
cleaning and coating in separate stages, each
followed by a water rinse. Parts can be cleaned
and coated in 2- to 5-minute time cycles if some
form of agitation is used. Cleanedcoater processes can be operated in the temperature range of
90°F (32°C) up to 160 to 170°F(71 to 77°C). The
operating temperature is normally controlled to
meet the cleaning requirements of the soils
encountered, and is influenced by the amount and
type of agitation available. Typical concentrations
used in such processes are for powder materials 2
to 4 oz/gal(l5 to 30 g/L)and for liquids 2 to 5%.
Power spray washers are the most widely used
method of applying an iron phosphate coating.
Nine-stage: clean, rinse, clean, rinse, treat,
rinse, post rinse, recirculated D.I., fresh D.I.
The user, working with the chemical supplier,
must decide how many stages will be needed,
depending upon a great many factors, including
the nature and amount of soil, the rinsing and/or
draining geometry of the ware, and the quality
levels required.
When a typical monorail conveyor system is
used, the cleaner stages will require 45 to 60
seconds; treatments require 45 to 60 seconds; and
rinse/post rinseA3.1. 15 to 30 seconds. Iron
phosphate treatments can be accelerated to
produce good quality coatings at various temperature ranges. Low temperatures will be 98 to 120°F
(32 to 49°C) and hot processes will be 140 to
160°F(60 to 71.1"C). As the line is placed in
operation, there is some thermal carryover, and
rinse temperatures can approach to within 20 to
37
Chapter 4
sludging and scaling, this method of application is
not used.
40°F(1 1-22°C)of the temperature of the preceding stages. Typical iron phosphate cleaner/coaters
are operated at 1 to 3 odgal(8 to 23 g/L)nominal
concentration, depending upon temperature
available, cleaning time, and soil condition.
Typical iron phosphate treatments are operated at
3 to 5%, or at a typical 10 ml titration.
Proprietary zinc phosphate coating compositions are available for immersion application;
these will deposit a paint-base-quality coating of
150 to 300 mg./sq. ft. Typical immersion systems
are composed of five stages: clean, rinse, zinc
phosphate treatment, rinse, post rinse. Immersion
zinc phosphate coatings can be developed in 2 to
5 minutes. Temperature requirements of 90 to
160°F (32 to 71"C)are needed, depending upon
the particular process. Typical concentration is 3
to 6%.
Zinc Phosphate
A great number of different proprietary compositions can be used to produce a zinc phosphate
conversion coating on steel surfaces. These
products are acid solutions containing zinc;
dihydrogen phosphate in aqueous solution; one or
more accelerating agents, typically zinc nitrate,
with or without tankside additions of nitrite; and
one or more modifying agents, grain-refining
agents, coating weight control agents, etc.
As with iron phosphate processes, power spray
washer application accounts for the largest
proportion of paint-base zinc phosphate treatment
processing. Depending upon a great many factors,
including the nature and amount of soil, the
rinsing and draining geometry of the ware, and
the quality levels required, as few as five stages
and as many as nine stages may be needed. Using
a typical monorail conveyor system, the times
utilized will be:
Proprietary zinc phosphate coating compositions have been formulated which can be brushed
on clean steel surfaces to produce a zinc phosphate coating. The surface must be precleaned
prior to the application of the conversion coating,
by any of the usual methods. The brush-on
treatment is applied, the coating is allowed to
develop, and the ware is then rinsed with fresh tap
water and dried. The brush-on treatment will
produce an acceptable coating in 2 to 5 minutes.
Compositions are typically applied unheated, to a
surface at room temperature. Since these are
mostly proprietary formulations, the
manufacturer's instructions must be followed to
determine concentration. It is technically possible
to produce a zinc phosphate coating on steel by
applying the coating product through a steam gun.
Due to the poor overall quality produced and the
short life of the operating equipment, as well as
Cleaner Stage:
45-60 seconds
Treatment Stage:
45-60 seconds
Post Rinse/Rinse/D.I.:
15-30 seconds
Zinc phosphate treatments can be accelerated
to operate and produce good quality paint-base
coatings at temperatures as low as 80 to 90°F(27
to 32°C). Typical zinc phosphate treatments are
operated at 3 to 6% concentrations, or at a typical
titration as low as 10 ml and as high as 25 ml,
depending upon the quality requirement and
particular proprietary treatment.
38
METHODS OF APPLYING POWDER
COATINGS
4. Other electrostatic application methods,
There are many methods available for applying
powder coating materials, with selection based on
such factors as:
including discs and powder coating tunnels.
FLUIDIZED POWDER BED
Product characteristics, including size of the
part being coated, type of coating to be
applied, desired or specified ultimate film
thickness, reason for applying the coating,
etc.;
PROCESS
This method of applying powder coatings was
introduced in the United States in the late O OS,
and was the first powder coating process used in a
production mode. In this process, a preheated
part is immersed into a fluidized powder bed.
The action of heat, and the powder coming into
contact with the part, result in the powder being
melted and adhering to the heated part. Material
coating thickness on the part is determined by the
part’s temperature and the length of time the part
is in the fluidized powder bed. Attaining the
proper cure of the deposited powder may require
post-heating the part, if the residual heat is not
sufficient to cure and flow the powder that
adhered during the immersion process. (Figure 51.)
Production quantities;
Number of colors used;
0
0
Means of conveying the part through the
coating process, line speed, and racking
configuration;
Means of applying the powder-automatically, manually, or a combination;
Available plant space; and
Capabilities of available powder coating
equipment in meeting immediate and future
needs.
Construction
The fluidized bed is constructed as a twocompartment container, or tank, with the top open
so that parts to be coated can be dipped into the
powder fluidized within the bed. The upper
compartment is used for storage and fluidization
of powder. A lower compartment serves as an air
plenum where compressed air is regulated and
dispersed through a porous membrane separating
the two compartments, as illustrated in Figure 52.
Available methods range from simple fluidized
beds to more sophisticated electrostatic spray
processes. Each has particular advantages for
specific coating applications. Generally, the most
widely used and accepted powder application
methods fall into the following categories:
1. Fluidized powder bed process.
2. Electrostatic fluidized bed process.
The size of the fluidized bed depends on the
size of the part to be coated. In the fluidized bed,
3. Electrostatic powder spraying.
39
Chapter 5
Figure 5-1. Note that the part temperature is the only thing that is important. The temperature in the oven serves
only to heat the part.
.
POWDERLEVEL
(UNFLUIDIZED)
STEP 1
\ POROUS MEMBRANE
STEP 2
FLUIDIZING AIR
STEP 3
Figure 5-2. Fluidized bed.
the size of the tank and the volume of material,
must allow the part to be immersed below the
upper level of the fluidized powder. Proper
fluidization is governed by even distribution of air
through the porous membrane, or plate, creating a
lifting effect and agitating the powder material, as
is seen in the final step. Mechanical vibration of
the container is sometimes used to enhance
fluidization and reduce the possibility of air
channeling and powder clumps. An important
aspect of proper fluidization involves the quality
of the compressed, or turbine blown, air introduced to the fluidized bed. Clean, dry air is an
absolute must. Oil, water, or pipe scale contami-
nants within the air supply would result in blocking, and possibly rupturing, the porous membrane, as illustrated in Figuqe 5-3.This results in
uneven fluidizing distribution, which will ultimately affect the finish of parts being coated.
Contamination of the powder material is also a
distinct possibility in cases where clean, dry air is
not used.
The fluidized bed should be constructed with
smooth walls that are virtually free of seams,
flanges, or other protrusions coming into contact
with the powder material. The porous membrane,
or plate, can be constructed of any available
40
Methods of Applying Powder Coatings
consumed on parts. A constant feed, conforming
to production usage, is ideal, though not essential.
Figure 5-3. Blocked diffusion.
I
porous material. Size and thickness of the
membrane should be sufficient to support the
volume of powder material and provide complete
fluidization to the interior of the porous bed.
Electrical conductivity of the material used in bed
fabrication is also important to help eliminate
static charge. The container should be adequately
grounded, to “bleed” off any static charge created
by the frictional movement between the powder
material and the container. Venting the porous
bed into a duct is sometimes necessary for
exhausting any powder particles that might escape
the container. This venting is usually designed to
ensure a clean, safe working environment.
1
Figure 5-4. Fluidization. The “dead dense powder
at the bottom leads to stratification.
Bed Density
Density of a porous bed results from the nature
of the powder itself and the operating conditions.
In many circumstances, it may be varied arbitrarily to produce desired results. However, in
deep beds it can become a critical factor.
The density of a particular powder may be
measured, of course, by its weight per volume.
That measure is, however, not sufficient to
determine how it will fluidize. The best way to
evaluate density in an operating bed is to measure
percentage of expansion from static to porous
condition. Correlating these data with previously
determined standards indicates whether a particular powder will fluidize throughout a reservoir
without requiring too much air or without exhibiting sluggishness in the tank, as shown in Figure
Fluidization
Every powder coating material resin possesses
its own peculiar fluidization characteristics. The
amount of air required per cubic foot, density of
the bed, physical conditions at the top and bottom
strata, all vary with the particular formula. In all
cases, however, it is essential that the bed be
completely aerated from top to bottom.
5-5.
A layer of “dead”4ense or compactedpowder immediately above the porous membrane
may lead to progressive stratification that ultimately affects the coating results achieved at the
dipping sector. This concept is illustrated in
Figure 5-4.
This may be done simply by calibrating a small
fluidizer to measure the percentage of expansion
attained by a given amount of powder at the point
of optimum fluidization.
Since all fluidizable powdered resins, thermosetting or thermoplastic, exhibit a particle size
distribution resembling a bell-shaped curve, it is
significant that the bed’s composition changes
For this reason, and others, it is desirable to
make powder additions to the bed in frequent,
low-quantity increments to replace powder
41
Chapter 5
during operation. There is a selectivity in deposition; the fines are removed from the bed at a
higher rate than are the coarse particles.
I
POWDER LOAD
II
EXPANSION
Basically, only the electrostatic field produced
in this process is used to apply the powder
material. In contrast to electrostatic spray methods, little air is used to control the volume and
velocity of the powder material. Figure 5-6
illustrates this point.
I
Applications
The electrostatic fluidized bed is ideally suited
to substrates that have a relatively small vertical
dimension, such as flat sheets, expanded metal,
wire mesh, screen, wire, cable, tubing, and small
parts on a conveyorized line. The electrostatic
fluidized bed offers the advantage of being able to
coat some objects, such as flat sheets, on one side
only; the conventional fluidized bed requires
dipping of the entire part. The electrostatic
fluidized bed includes an integral control panel
and utilizes the same power-supply unit as the
manual electrostatic spray system.
-Figure 5-5. Powder load and expansion.
ELECTROSTATIC FLUIDIZED BED
PROCESS
Although the phrase "electrostatic fluidized
bed" is used in describing this process, the
"fluidized bed" portion of the system is of lesser
importance in the actual powder deposition than it
is in a dipping process. This concept does not use
a spray device or nozzle to apply the powder
material to the parts.
Preheating the part is not necessary with this
method. The electrostatic forces-negative or
positive to ground-ause
deposition of the
powder on either hot or cold surfaces. If the
surface is hot, the deposited powder will sinter
and adhere upon contact. If it is cold, the electrical charge on the particle will hold it in place until
it is post-heated to cause flow. An advantage of
post-heating is that powder can be selectively
removed, reducing or eliminating the need for
masking.
In this process, the combination of the fluidized bed and electrostatic charging medium
creates a cloud of charged powder particles absve
the powder bed. As in any fluidized bed, the
powder material is fluidized by introducing
compressed air through a porous membrane into
the powder bed. Depth of powder material in the
bed is usually 1/2" to 1".
Powder Characteristics
Factors affecting the application of powders in
this process involve the types, sizes, and particle
shapes of the powder material to be used. Generally speaking, types of powders proven acceptable
for the electrostatic fluidized bed process are:
An electrostatic charging medium charges the
air used to fluidize, when high voltage DC
potential is applied. Ionizing this air electrostatically charges the powder particles as the fluidizing air moves upward through the powder bed.
The charged powder particles, having similar high
electrical charges, thus repel each other and move
upward. Aided by the fluidizing air, a cloud of
charged powder forms above the upper surface of
this powder bed. Grounded parts can be conveyed through or immersed into this cloud.
Epoxy;
Epoxy/polyester hybrids;
Polypropylene;
Polyethylene;
42
Methods of Applying Powder Coatings
#
. . .'.
*. ... . . ... ... .
r
. *
,
.-. . . .. .. B . . ? . - -. ..
. y * . .. . . .
... ..'
. . ..
. . .. *. . . .
. .
. .
.. . . . . ..
. .. . . , .. . . ....
.. . .. . .. . . . . .
. .
.. . .. .
..
.
.. .
j
LOW VELOCITY
CLOUD OF
CHARGED PARTICLES
.
.
e .
*
*
'
GROUNDED PART
AT ROOM TEMP
'
'
,
,
.
*
'
a
.
I
Figure 5-6. Electrostatic fluidized bed. (Source: Electrostatic Technology, Inc.)
Acrylic;
the powder bed and those with a remote charging
media.
Polyester;
Locating the charging electrodes in the powder
bed is effective, but it poses the hazard of arcing
between the electrodes and the grounded substrate. The principal method for achieving
powder charge is to locate the charging medium
in a plenum below the powder bed, eliminating
the possibility of arcing. Inlet air is ionized and
the charge is passed to the powder during fluidization. The voltages employed with either
method nominally are between 30 and 100
kilovolts. In most cases, negative polarity is used.
Positive polarity may be of some benefit with
nylon.
Nylon; and
Engineering thermoplastics such as PFA,
PPS, PEEK PVDF.
Fluidization and transfer efficiency also are
influenced by the particle size distribution.
Equipment Characteristics
There are basically two styles of electrostatic
fluidized beds, those with electrodes immersed in
43
Chapter 5
The amount of current from high-voltage
sources used with this technique is in the range of
200 microamperes. The current draw is independent of the amount of material being charged. It
is related only to the proximity of the ground
object to the charging media and to the resistive
network used in the circuit. The amount of
voltage applied to the charging medium affects
the density of the charged powder cloud. The
higher the voltage, the denser the cloud, and the
greater the amount of deposited powder.
100 kilovolts maximum and the maximum output
energy (joules) when the unit is shorted to ground
must not be greater than the ignition energy of the
particular materials being applied. Also, output
regulation of the supply-rate of fall of the output
voltage with current draw increase-must allow
rapid overcurrent protection.
An important consideration when using the
exposed electrode bed with preheated parts is that
the parts being coated must not be heated excessively in the preheat oven. When the parts being
coated are too hot, gasses and vapors can be
released within the bed causing the air between
the part and the electrode to become conducting,
creating a sparking condition between the part
and source.
Other determining factors related to the amount
of powder deposited are exposure time and
distance above the upper surface of the aerated
powder bed of the product to be coated. It will be
evident that the deposition rate lowers as the part
is moved away from the charged powder.
It is good practice to employ an exhaust system
with the exposed electrode type bed. Top exhaust, entrance and exit port exhaust, or perimeter
exhaust are all applicable techniques. Velocity of
exhaust air should be a minimum of 50 F’PM
(15.24 dmin). The same collection and recovery
devices employed with electrostatic spray can be
used with this technique.
A common concern with all electrostatic
deposition techniques is the “Faraday Cage”
effect, which can cause a poor powder build,
especially inside acute angles. The electrostatic
fluidized bed has been employed successfully in
challenging applications such as coating the slots
in motor armatures for insulation purposes.
Design and Application
With the electrostatic fluidized bed, powder
particle-sized classification is not experienced to
any great degree so long as deposition takes place
near the upper surface of the bed. However, the
further one moves above this point, the more
pronounced the classification becomes.
Several electrostatic fluidized bed designs are
available, but some may be controlled by various
patents. All use the same basic concept of
employing the fluidized portion as a material
supply reservoir for the charged powder cloud.
An arrangement typical of these devices is shown
in Figure 5-7.
Safety Considerations
Safety concerns vary, depending upon the
design of the coating unit. The electrostatic
fluidized bed design {with the charging medium
in the plenum} is known to be a safe design, with
no potential for an arc between the charging
medium and a grounded part.
CHARGED
wwDEn
CLOUD
WWDER
CHAMBER
AERATED
mwoER
BATH
DIFFUSER r u i E
Definite safety factors must be carefully
considered when using an electrostatic fluidized
bed that has electrodes in the powder bed. Since
the charged cloud density is extremely high,
design of the high-voltage source is very important. The output voltage should be approximately
PLENUM
cw”n
Figure 5-7. An electrostatic fluidized bed design with
electrodes in powder bed.
44
Methods of Applying Powder Coatings
The plenum chamber, diffuser plate, and power
chamber must all be made of an electrically
insulating material. Normally, rigid PVC or
polyester reinforced fiberglass is used for the
plenum and powder chamber and either highdensity porous polyethylene or porous ceramic for
the diffuser plate. The supporting framework can
be of steel construction.
Sample Application
The electrostatic fluidized bed is used for a
variety of substrate types. This includes linear
materials such as small parts, wire, cable, screen,
and coil products such as steel. The electrostatic
fluidized bed in a production environment is
usually combined with other processing systems
into an integrated powder coating machine. An
example of such a machine is illustrated in Figure
5-8.
The fluidizing air used in conjunction with this
process must be dry and free from such contaminants as oils, silicon, etc. Normally air at a
pressure of 2 to 10 PSI will be required at a rate
of approximately 5 CFM per square foot of
diffuser plate. A suitable air drier must be used in
conjunction with the air source, either compressor
or blower.
The illustrated machine is designed to coat
motor armatures with an insulating film of epoxy.
Parts are loaded onto a conveyor that rolls the
armature forward through the system. This
ELECTROSTATIC
COATER
I
Hi
POWDER MANAGEMENT
NDUCTION HEATER
CURE ZONE
I
AIR COOLING,
I
ROTO-FLO CONVEYOR
x
Figure 5-8. Electrostatic fluidized bed in a production environment is usually combined with other processing systems
into an integrated powder coating machine.
45
Chapter 5
There are other devices to enhance the operation of these basic components.
method of conveyance therefore presents a
constant distance from the powder bed to the
surface to be coated, on this cylindrical part. As
the part is coated at essentially room temperature,
the powder can be selectively, automatically
removed from surfaces not to be coated, eliminating the need for masking. To attain high throughput rates, induction heaters are used to quickly
raise the temperature of the part. The part is then
passed through a convection zone for cure
completion, and finally air cooled.
In the operation of an electrostatic powder
spray system, powder is siphoned, or pumped,
from a feeder unit through a powder feed hose to
the spray gun(s). Spray guns direct the powder
toward the part in the form of a diffused cloud.
Propelling force is provided both by air that
transports powder from the feeder unit to the
spray gun, and by the electrostatic charge imparted to the powder at the gun. Electrostatic
voltage is supplied to the spray gun by a source
designed to transmit high-voltage, low-amperage
electrical power to an electrode(s) attached to the
spray gun. As the diffused, electrostatically
charged powder cloud nears the grounded part, an
electrical field of attraction is created, drawing the
powder particles to the part and creating a layer of
powder. Overspray-or powder not adhering to
the part-is collected for re-use or disposal. In
the collector unit, powder is separated from the
conveying airflow. Collected powder is then
automatically or manually recycled back to the
feeder unit to be resprayed. Air is passed through
a filter media device into a clean-air plenum and
then through a final, or absolute, filter back into
the plant environment as clean air. The coated
part is then carried from the application area and
subjected to heat, which results in the flowout and
curing of the powder material.
ELECTROSTATIC POWDER SPRAY
COATING
Electrostatic spraying is the most widely used
method of applying powder coating materials. Its
growth is increasing at an impressive rate. Developed in the mid O OS, this process is the most
efficient means of applying coatings and finishes
in a short time. However, acceptance of powder
coating in general was initially very slow in the
US. In Europe, the electrostatic powder spray
concept was accepted more readily, and the
technology moved much faster there than elsewhere in the world. However, many advances
were made in both powder materials and application equipment available to U.S. manufacturers.
These advances generally involved problems
associated with electrostatic powder spray coating, as well as improving functional operations of
system components. As a result, there is a wide
variety of electrostatic powder spray coatings
systems available in the United States today.
Economic Advantage
With electrostatic powder spray, up to 99% of
the powder overspray can be recovered and
reapplied. Material loss experienced with powder
is minimal in comparison to liquid coating
systems.
To apply powder coating materials with the
electrostatic powder spray process, five basic
pieces of equipment are needed:
Powder feeder unit;
In addition, in most cases powder provides
one-coat coverage without runs and sags on the
finished part. Applying a primer coat prior to the
finish coat is unnecessary, reducing time and
labor required by multicoat liquid systems.
Electrostatic powder spray gun, or equivalent
distribution device;
Electrostatic voltage source;
Reduced fuel cost in curing powder often
results from the use of smaller ovens, shorter
oven times and, in some cases, lower oven
Powder recovery unit; and
Spray booth.
46
Methods of Applying Powder Coatings
temperatures. There is no need to heat or temper
booth makeup air since air is returned to the plant
environment as clean air.
part (before heat curing) and recoated. This can
eliminate the labor and costs involved in stripping, rehandling, recoating, and recuring rejected
parts.
Other cost savings, including lower clean-up
costs, can be attained with powder. There is no
need to mix, recover, and dispose of solvents
when coating with powder. Usually, no solvents
or chemicals are used in cleaning either the
powder application equipment or the spray
booths. Since air and vacuum cleaners are
generally all that is required for cleaning with
powder, labor and cleaning materials are reduced,
and disposal of hazardous paint sludge is eliminated.
Users are finding that the powder spray coating
process is easily automated. It can utilize
automatic gun movers, contouring mechanisms,
robots, and stationary spray gun positioning.
Total production time can often be reduced, or
production volume increased, with powder spray
coating. Eliminating various steps required with
the liquid coating process can result in a more
efficient finishing line.
EQUIPMENT
A large percentage of liquid coatings is comprised of sometimes toxic and flammable solvents
that are lost in the application process. Shipment,
storage, and handling costs of solvents are usually
very expensive. With powder, costs involving air
pollution control equipment, flash-off time, and
solvent waste disposal are virtually eliminated.
Most electrostatic powder spray coating
systems are comprised of four basic pieces of
equipment-the feed hopper, electrostatic powder
spray gun, electrostatic power source, and powder
recovery unit. A discussion of each piece, its
interactions with other components, and the
various styles available is necessary to understand
the functional operation of this process.
Eliminating solvent usage also can reduce fire
insurance requirements as well as rates paid to
maintain fire insurance protection.
Powder Feeder Unit
Finally the applied cost per mil per square foot
of film is equivalent to, or lower thari, liquid
coating costs in most cases.
Powder is supplied to the spray gun from the
powder feeder unit. Usually powder material
stored in this unit is either fluidized or gravity-fed
to a pumping device for transport to the spray
gun(s) (Figure 5-9). Newly developed feed
systems can pump powder directly from the
storage box.
Ease of Application
The consistent finish characteristics and
electrostatic “wraparound” realized in powder
spray applications help reduce the need for highly
skilled operators. In addition, there is no viscosity balance to maintain when coating with powder. Powder materials come “ready to spray”
from the manufacturer.
The pumping device usually operates as a
venturi, where compressed or forced airflow
passes through the pump, creating a siphoning
effect and drawing powder from the feed hopper
into powder hoses or feed tubes, as shown in
Figure 5-10. Air is generally used to separate
powder particles for easier transporting and
charging capabilities. Volume and velocity of the
powder flow can be adjusted.
No flash-off time is required with powder. The
coated part can be transported directly from the
spray area to the oven for curing. Reject rates can
be reduced, as can costs involved in reworking
rejected parts. Runs and sags are usually eliminated with the powder coating process. Insufficient or improper coating can be blown off the
In most cases, the feeder device utilizes either
air, vibration, or mechanical stirrers to aid in
47
Chapter 5
,--.
"..".-
THE HOPPER-VENTURI
FEEDER
THE FLUIDIZED BED-VENTURI
FEEDER
Figure 5-9. Typical types of powder feed devices. (Sources: Ransburg-GEMA and Volstatic, Incorporated.)
gun(s), are designed similar to the fluidized bed
concept. Compressed, or forced, air is supplied to
an air plenum generally located at the bottom of
the feeder unit. Between the air plenum and main
body of the feeder unit is a membrane, usually
made of a porous plastic-composite material.
Compressed air passes through it into the main
body of the feeder unit, where powder material is
stored. The fluidizing action of the air results in
lifting the powder material upward, creating an
agitated, or fluidized, state (Figure 5-2). With
this fluidizing action, it is possible to control the
metering of powder siphoned from the feeder unit
by the attached, or submerged, venturi-style
pumping device (see Figure 5-9).
"breaking up" the powder mass. This action
results in much easier transport of the powder,
while assisting in regulating the volume and
velocity of the powder flow to the spray gun(s).
Independent control of powder and air volumes
aids in attaining the desired thickness of coating
coverage. The powder feeder is capable of
providing sufficient material to one or more
electrostatic spray guns several feet away. Powder feeders are available in many different sizes,
with selection depending on the application,
number of guns to be supplied, and volume of
powder to be sprayed in a specified period of
time. Generally constructed of sheet metal, the
feeder unit can be mounted adjacent to, or even be
an integral part of, the recovery unit.
When gravity feed-type feed units are used, the
operation involves a conical or funnel-shaped unit
in which powder material is stored. Pumping
Feeder units, which utilize fluidizing air to
facilitate pumping of powder material to the spray
48
-
-
Methods of Applying Powder Coatings
devices attached to this type of feeder unit usually
are of a venturi-type pump. In some cases,
vibration or mechanical stirrers are used to
enhance powder siphoning by the venturi effect
produced by the pumping device. Powder is
gravity fed to the pumping devices, and fluidizing
of the powder is not necessary. Again, see Figure
5-9. Powder may also be delivered directly from
powder boxes or containers using a double-well
siphon tube, which provides enough local fluidization to allow uniform delivery.
within the closed loop of powder delivery, spray,
and recovery (Figure 5-1 1).
OPERATION
Feeders
Since the feeder is the first step in delivering
powder to the part, it must be well set up and
maintained. Poor operation at the feeder will
result in noticeably deteriorated performance in
the downstream equipment.
Sieving devices are sometimes used in conjunction with feeder units to screen out any dirt,
clumps of powder, and other debris, and to
condition the powder prior to spraying. These
sieves can be either mounted directly to or above
the feeder unit to facilitate easier flow of powder
Air used to fluidize and pump the powder must
be clean and dry. Oil, moisture, rust, and scale
can contaminate the powder and even block the
pores in the fluidizing plate. This in turn can lead
to poor fluidization or even a ruptured plate.
~
Powder Air Mix to
Spray Gun
-%A
I
Air Supply
Powder
L
Suction Air
High Density/Low Volume
Powder/Air MIX
Powder from
Fluid Bed
F;gure 5-10. Venturi cutaway view from side. (Source: GEMANolstatic, Incorporated.)
49
.
Chapter 5
dency to impact fuse is related to the velocity, the
directness of impact (blunt versus grazing impact), wall material, and the nature of the specific
powder. A good preventive maintenance program
is essential to a properly operated powder system.
CYCLONE VENT,
CYCLONE:RECLAIM
POWDER INTAKE-
Sieving devices mounted on the feeder unit
must be free of debris, and screens must be kept
clear of powder buildup. Proper venting of the
sieve is critical as the performance deteriorates if
there is much differential pressure across the
screen.
Powder Delivery Hoses
A key part of the powder’s path on its way to
the part being painted, the powder hose is often
overlooked. Hose routing should be as direct as
possible, with hoses trimmed to eliminate excess
length. Routing should also avoid sharp bends or
kinks. A bend radius of 9 inches (22 cm) is
considered good practice and will help reduce
wear, impact fusion, and pressure drop. Frequent
inspection for internal wear, external chafing, and
impact fusion is important.
Figure 5-7 7. Powder feed hopper with sieving device
attached. (Source: Nordson Corporation.)
The hopper must be properly vented to reduce
pressure buildup. Poor venting can affect fluidization, and can cause fugitive powder to leak into
the workplace, creating housekeeping and safety
problems. The hopper must also be electrically
grounded (earthed) to prevent static electricity
buildup. In fact the National Electric Code
requires that all “dead” (not an active part of a
circuit) metallic and conductive components in an
electrostatic spray system be grounded. See
NFPA-33.
A variety of material can be used in powder
hoses, the choice being based on flexibility, kink
resistance, freedom from tribocharging (static
buildup due to powder flow), resistance to impact
fusion, lack of chemical interaction with the
powder, and wear resistance. Since there are
many parameters to balance and many types of
powders available, there is no “best” material.
The feeder should be securely mounted or
designed in a manner that effectively prevents it
from tipping over and spilling powder into the
workplace.
Electrostatic Spray Guns
Electrostatic powder spray guns impart the
electrostatic charge to powder particles, and shape
and direct the pattern of those particles on the
way to the part. The gun controls the deposition
of powder on the parts through variations in gun
position, velocity and shape of pattern, and charge
levels.
Pumping devices used to convey powder from
the feeder to the powder spray gun(s) should be
inspected and cleaned on a regular basis. Parts
exposed to high-velocity streams of powder are
prone to wear and/or impact fusion. Worn parts
result in loss of performance and control of the
painting process. Impact fusion causes blockage
and reduced flow rates. Impact fusion is a
sintering process in which the powder grains
become fused together in hard, tightly bonded,
deposits on the walls of powder passages. Ten-
Powder spray guns are available in both
manual (hand-held) and automatic (fixed mount)
types, in internal and external charging, corona
charging with internal or external high-voltage
50
Methods of Applying Powder Coatings
supplies, and in triboelectric (friction charging)
types. All of these variations have their own
benefits and weaknesses, and their own role in the
painting of parts.
Faraday Cage is ultimately counterproductive
since the powder will be less well charged and
less able to deposit on the part if it does succeed
in getting into the cavity. Keeping the voltage as
high as possible and working on the aerodynamic
aspects of the transportation maintains a higher
charge level on the powder and results in more
efficient and effective coating of the cavity.
Internal and External Charging Guns
In these guns, powder is charged by ion
bombardment in a region close to the charging
electrode. A high voltage of 30-to-100 kilovolts
(kV) and usually negative polarity is applied to
the charging electrode. This voltage creates a
very strong electric field around the electrode,
which in turn causes a breakdown (ionization) of
the air around the electrode to form a corona
discharge and an ion current. The ions are
directed by the field to the powder particles,
which are bombarded by the ions, transferring
charge to the particles. The charged particles are
then carried to the parts primarily by air currents
and, to a lesser degree, by the electric field forces.
Once the charged particles come close enough to
the part being painted (within about 1 cm) the
attraction between the charged particle and the
grounded part causes the particles to effectively
deposit on the part.
The rate or thickness of deposition is controlled
by powder flow rate, position of the spray gun,
pattern used, velocity of the powder stream, line
speed, part geometry, and the level of charge on
the particles. Characteristics of the powder being
sprayed can also affect deposition. Important
factors are: type of material, mean size, and
probably most important, shape of the distribution
curve.
Internal Charging Guns
In an internal charge corona gun, the charging
process is by ion bombardment, the same as in an
external charge gun. However, while the charging electrode is referenced to the grounded part to
form the corona in an external gun, the internal
charge gun carries its own ground reference
internally. The results of this are little or no
external field forces, and little or no excess ion
current. The reduced field will help penetrate the
Faraday Cage (cavities) areas since the external
field provides a small diverting force that tends to
move particles away from the cavity. It is well to
remember that transporting powder into the cavity
is almost entirely an aerodynamic process. The
reduced ion current diminishes the tendency
toward back ionization, which leads to smoother
surface finishes with fewer “cratering” or “starring” defects. Reduction of back ionization also
reduces self-limiting effects of electrostatic
powder coating and allows heavier film builds in
the 10-to-15 mil range, which is difficult to
achieve in external charge systems.
Regardless of the type of gun being used, the
transport of well-charged particles to within 1 cm
of the surface is essential for efficient and effective painting.
The external corona gun is by far the most
common type in use today, and is well suited for
flatware and high line speeds with good uniformity and high transfer efficiency.
Performance on Faraday Cage areas has
improved over the years, with attention to careful
pattern control and gun placement. The much
more effective flat spray pattern is now used by
approximately 80% of all automatic guns.
Since performance on Faraday Cage areas
depends on getting powder into the cavity,
success is ultimately determined by pattern
control and aerodynamics-the transportation
means, rather than concentration on the electrostatic effects. The older conventional wisdom of
turning the gun voltage down to help penetrate a
Internal charging corona guns tend to require
more frequent maintenance than other types
owing to the necessity of keeping their ground
reference clean and free of powder, and because
of their complex and often fragile components.
51
Chapter 5
flexible, lightweight cable. The internal generator
steps this low voltage up to the required 30-to-100
Kv in the gun. In an external supply, the high
voltage is generated externally and carried to the
gun in a thicker, stiffer, heavier high-voltage
cable.
Triboelectric Charging Guns
Triboelectric guns have no high-voltage
powder supply. They work by arranging the
internal geometry so that air, when conveyed,
rubs powder grains on the walls of the gun,
transferring charge by frictional means. Polarity
of charge is almost always positive. If tribo guns
are used with corona guns, the corona guns
should be obtained in positive polarity as well.
Placing the generator in the gun has benefits
with regard to the cable, but adds weight to the
gun, and-in hand guns particularly-places
restrictions on the designer to keep the highvoltage generator light and compact. Also, in
applications where collisions with parts or
machinery can occur, or in high-temperature
environments, there is potential damage to a highcost component if the generator is in the gun.
Conversely, the external supply has no practical
restrictions on size or weight and it is protected
from damage.
Generally, deposition rates are lower for tribo
guns, requiring more guns per line. However,
some of the better modem designs can achieve
flow rates and charge levels comparable to
external corona charging guns, given a favorable
powder. Since the charging process depends on
inertial forces bringing the particles in contact
with the walls, and since the charge transfer is
related to the relative chemical compatibility of
the powder and the wall material, the process is
sensitive to both the particle size distribution and
the chemistry of the powder being sprayed. Many
powder companies have addressed this issue, and
lack of suitable tribo powders is less of a problem.
There are, however, powders that cannot be
sprayed via tribo-for example, some metallic
dry blends. Also many colors may not work
unless specifically formulated for tribo application. It is imperative that the user coordinate with
his or her coating suppliers when tribo guns are
used.
Skilled design engineers can make both
systems deliver equal performance in painting parts.
APPLICATIONS FACTORS
In addition to proper selection and set up of
feeders, pumps, hoses, and spray guns, a number
of other factors can affect the end result. Foremost is the role of the powder itself, especially the
powder’s particle size distribution.
Accumulated experimental evidence and
experience indicate that the shape of the distribution curve, rather than the particle size per se, is
the best indicator. A histogram of the particlesize distribution should show a narrow peaked
shape. Broad, flat distributions with large percentages of both coarse and fine particles will
show a number of undesirable properties, including poor fluidization, lower transfer efficiencies,
and rapid buildup of fines. It is believed that the
presence of coarse particles inhibits effective
charging of finer particles. Where smoother, high
gloss finishes are desired, finer grind powders are
often required. However, it is important that not
only the mean size, but the shape of the distribution curve, be adjusted.
Since there is no external electric field and no
excess ion current, the tribo gun has all the
benefits associated with internal charging corona
guns, but without their stringent maintenance
requirements and internal complexity.
Internal Versus External Power
Supplies
In corona charging guns (either internal or
external charging), the high-voltage power supply
may be located either internally or externally.
There are tradeoffs inherent in both designs.
The internal supply receives low-voltage power
from an external control unit through a thin,
52
Methods of Applying Powder Coatings
statics that transports powder around to the back
edge flat panel or the backside of a round tube;
aerodynamic turbulence provides the transportation.
In tribocharging guns, the powder plays a
critical role operating the gun. While dry-blend
additives can enhance tribocharging, best practice
is to alter the melt-mix to achieve a homogeneous
material.
Managing booth air flow, and selecting the
correct nozzle or pattern shaping device, advantageous gun placement, and pump settings all
contribute equally to the electrostatic effects.
In the past, conductivity was thought to be an
indicator of chargeability in powders. Today,
there is no evidence that this is the case.
The tendency of powder coating to limit the
film build and promote more uniform coatings
has been shown by John Hughes of the University
of Southampton, England to be caused by back
emission of ions from the deposited film. These
ions act to discharge the incoming powder and to
prevent its deposition. Back ionization is also
responsible for orange peel, cratering, and starring
defects in the film. Back ionization can be
overcome by moving the guns further away,
reducing the voltage, using an internal charge gun
(either tribo or corona), or by applying the
powder in thin layers with 10-to-15 seconds
“relaxation time” between applications. Powder
chemistry and particle-size distribution are also
involved.
OPERATION CONDITIONS
For electrostatic powder spray guns to function
properly and safely, the following conditions
should be maintained:
If metallic, fixed powder spray guns must be
adequately grounded at their points of support
to reduce the possibility of static charge
buildup on the gun and the discharge of this
static charge to a part or component in the
spray area.
Manual powder spray gun operators must be
adequately grounded (usually through the gun
handle) to prevent buildup of a static charge
on the operator’s body during spray operations.
Although it is true that the powder spray
process is dependent on electrostatics, and
without charged powder we do not have a process, the powder spray process is only about 50%
electrostatics. The other half depends on air flow
to shape the patterns and transport charged
powder to the parts.
Powder spray gun parts that come into
physical contact with moving powder must be
inspected and cleaned on a regular basis.
Parts contacting moving powder are prone to
wear (if the powder material is abrasive) at
high velocity and impact fusion. Worn parts
result in poor control of powder flow, accentuated impact fusion, and more frequent
cleaning. If a part is obviously worn, it
should be replaced.
Faraday Cages are areas where there is, by
definition, no electric field from an external
source. Under close examination, it can be seen
that the cavities referred to as Faraday Cages are
also zones where it is difficult to achieve air flow.
These are difficult areas to get powder into for
aerodynamic reasons. If you cannot blow powder
into a space, that space cannot be coated. The
older conventional wisdom of “reducing the
voltage to overcome a Faraday Cage” is actually
treating the symptom, not the underlying disease.
Electrostatic powder spray guns (manual and
automatic) should be checked periodically to
determine the level of electrostatic charge
being imparted to the powder material. The
lack of, or decrease in, expected electrostatic
charge indicates a problem in the electrostatic
system and should be corrected as soon as
possible. To reduce the possibility of electri-
“Wrap” is also spoken of as an electrostatic
phenomenon. And to be sure, without charge
there is no evidence of wrap. But it is not electro-
53
Chapter 5
cal shock, troubleshooting guides should be
utilized when inspecting or repairing any
component within the electrostatic system.
The electrostatic power unit must be adequately grounded.
e
With fixed or automatic powder spray guns,
interlocks should be used to rapidly deenergize the high-voltage elements involved
with electrostatic spray under any of the
following conditions: stoppage of ventilating
fans or failure of ventilating equipment from
any cause, stoppage of conveyor carrying
goods through the high-voltage field of
electrostatic spray, and other conditions as
prescribed by regulatory agencies.
Electrostatic power units must be installed in
a manner consistent with conditions specified
by regulatory agencies.
Electrostatic power units should be inspected
periodically to determine the level of electrostatic kilovoltage output.
0
54
Troubleshooting guides provided by the
manufacturer should be strictly followed, to
reduce the possibilities of electric shock.
POWDER SPRAY BOOTHS
AND RECOVERY SYSTEMS
Recovery systems can be as simple (Figure 61) or as sophisticated (Figure 6-2) as the application requires. Whatever the application, a proper
design based on a sound understanding of the
applicable physics and engineering fundamentals
is essential to the efficiency and longevity of the
system operation.
Efficient recovery of oversprayed material is
one of the most important aspects of an electrostatic powder spray system. Of the quantity of
powder leaving a spray gun, 30 to 90% adheres to
the part. The oversprayed material must be
collected effectively and recycled by a recovery
system in order to achieve the high system
efficiencies characteristic of powder coating
applications.
SPRAY BOOTHS
Air movement is the primary tool in virtually
all methods of collecting over-sprayed powder
materials. Collection systems must address
several important requirements:
An important element of the spray booth is the
material of which it is constructed. Typically,
booths are constructed of polypropylene, stainless
steel, or coated steel panels. Some booths are a
combination of materials. One unique design
utilizes retractable polyethylene sheeting. Selection criteria include: effect on transfer efficiency
(whether the booth material is conductive or nonconductive), cleanability (how strongly the
powder is attracted to the booth wall), visibility in
the spray area (translucence of the material), as
well as strength, durability, and repairability
given the size of the booth and specifics of the
application.
Containment of overspray in order to limit
worker exposure and minimize housekeeping;
Efficient separation of powder from air
volumes;
Ease and time of color change;
Control of air movement in spray zones for
augmentation of application transfer efficiencies;
Parts openings in a spray booth should be sized
properly to allow clearance for the largest part to
be coated. Openings for automatic and manual
spray stations should allow sufficient access and
be properly positioned for best coating efficiencies. Accommodations must be made for the
induced air flows associated with spray equipment. Booth walls around such,openings should
be configured and contoured appropriately to
minimize “dead zones’’ in the spray area where
powder can build up.
Minimization of operational noise levels for
worker protection;
Safety and insurance agency approval regulations, particularly in the areas of fire and
explosion prevention;
0
Ease and time of installation; and
Comfort and convenience for the worker
during system operation.
55
Chapter 6
/
c
Figure 6-1. Batch booth with no quick color change.
Gravity-assisted Booths with Cyclone
Recovery
In gravity-assisted recovery booths, a portion
of the overspray returns directly to the feed
hopper by gravity without entering the reclaim
system. This minimizes the amount of reclaim
powder generated within the system (Figure 6-4).
This booth design lends itself to be used with a
variety of products (e.g., facets, lawn mower
decks, housings, castings, etc.). The products
coated are usually arranged on a rack or “hanger”
used in conjunction with an overhead conveyor.
In this system, about 50% of the overspray falls
back to the feed hopper by gravity. The balance
is collected through an extraction duct to the
reclaim system. In this system, the reclaim
system is a virtually self-cleaning cyclone separator with reclaim recovery efficiencies from 90 to
95%. The small fraction of powder remaining in
the air stream, from the cyclone, is separated in
the final filter before the air is returned to the
powder coating room. Figure 6-3 is only one of
several possible configurations of this equipment.
Typically, painted cold-rolled steel or stainless
steel is used to construct the booth cabin walls.
The use of an efficient coating chamber with a
self-cleaning cyclone allows an unlimited number
of color changes without duplication of filtering
equipment.
Quick-color Change Booth with
Cyclone Recovery
This spray booth design has replaced booth
walls, metal and/or rigid plastic, with indexing
thin plastic sheeting (Figure 6-5). When a color
change is desired, pneumatic air motors index
clean booth walls. This eliminates manually
cleaning down the booth walls for a new color
56
Powder Spray Booths and Recovery Systems
...
Figure 6-2. Conveyorizedbooth with multiple color modules that allow quick color change.
Figure 6-3.Basic components of an electrostatic sray coating system. (Source: Volstatic, Incorporated)
57
Chapter 6
Figure 6-4. Variations of gravity-assisted recovery type booths (Source: Volstatic, Incorporated)
This style spray booth can be used with either a
cyclone reclaim system or a cartridge module
system. There is a variety of products used with
this booth, and an overhead conveyor is used to
transport the product through it.
Rigid Booth with Cyclone Recovery
In contrast to gravity-assisted booths, this
booth sends all the other spray through the
cyclone collection system and the detached
powder feed system.
Other Booth Technologies
Horizontal Coating Booth. Typically, this
booth is used to powder coat a product that
requires coverage on all surfaces except one. The
surface area to be masked is placed face down on
the conductive conveyor belt, which not only
transports the parts through the spray booth, and
provides the necessary ground for the part. It also
serves as an automatic mask. When appropriate,
this system helps to eliminate the cumbersome
tasks of masking and racking. Parts may be
manually or automatically loaded and unloaded,
and even use some type of automatic vibratory
feeder for loading (Figure 6-6). This booth
Fjgure 6-5.This spray booth, whose ceiling, walls, and
floor are made of indexing thin plastic sheetingl
requires less downtime for booth cleaning.
and allows the spray booth to change colors in 1
to 2 minutes. Once the new plastic is in place,
operator openings and/or automatic gun slots are
simply cut out where needed.
58
-
._
Powder Spray Booths and Recovery Systems
Typical Layout:
Figure 6-6. Horizontal coating booth can help eliminate the cumbersome tasks of masking and racking.
design can be used with types of products such as
disc brake shoes, washers, nuts, etc.
booth are: oil filters, motor housing, cans, bulbs,
bottles, etc. Most of these parts are NOT complex
in shape and are rotated through the booth to be
coated.
Belt Booth. This is another type of application
booth employing a moving belt in the bottom of
the booth (Figure 6-7). Construction of this booth
system includes a fabric belt traveling in a
horizontal loop along the booth floor.
Oversprayed powder is drawn to the belt surface
by the airflow created by the booth exhaust
system. Powder particles trapped on the surface
of the belt are vacuumed off by a pickup head
located at the end of the booth. Once removed
from the belt, the powder is sent through the
reclaim system to be separated from the vacuum
airflow and prepared for re-use. Like some of the
other booths, this booth can be used with a variety
of products and with some type of overhead
conveyor.
COLLECTOR SYSTEM
The powder collection system must maintain
sufficient velocities through the booth openings to
contain oversprayed powder material. Accordingly, size of the collection system is dictated
primarily by square footage of the booth opening.
Systems for collecting, separating, and processing
over-sprayed powder typically incorporate one or
more of the following techniques. Two types of
cartridge spray booths are utilized, depending on
the size of the part being coated and the number
of guns.
Self-contained Booths
Chain-on-edgeBooth. This type of booth has
a cartridge filtration for recovering the powder
and is used to coat products passed through on a
spindle conveyor. The conveyor for this booth is
floor mounted and uses a pressurized shroud to
keep powder off the conveyor as the parts are
coated. Types of products using a chain-on-edge
One type of application system is a selfcontained spray booth and collector. Air used to
contain and recover oversprayed powder is
filtered through primary and final filters and
returned to the plant as clean air. Solid state
59
Chapter 6
I
Figure 6-7. This belt booth uses a moving belt to assist in recovering overspray powder material. After air from the
belt draws overspray to the surface, a vacuum pick-up transfers the powder to a cattridge-type collection device for
recycling.
Side Draft Spray Booth
controls for filter cleaning and powder recycling
provide fully automatic reliable operation.
In a side draft spray boothkollector (see Figure
6-8), the powder spray booth provides clean,
efficient, and flexible operation in manual or
automatic powder coating applications. Utilizing
cartridge filter technology, it provides high
material utilization and fast color change capability in a space efficient economical system.
The “downdraft” design of the booth locates
the filter/collector module directly below the
spray booth for maximum operating efficiency in
most coating applications. The downward air
flow allows oversprayed powder to naturally
wash down over the part, resulting in more
consistently uniform coating and improved
operating efficiency.
The booth has a rollaway,filter/collector
module design. Vertically mounted, highefficiency cartridge filters use reverse-pulsed air
to prevent excessive powder buildup and extend
cartridge life. With improved filter cleaning, the
module provides maximum air flow throughout
the coating operation for optimum system performance. The collector module’s quick disconnect
The downdraft design also provides more
efficient use of floor space and allows access to
either side of the booth for automatic and manual
spray gun stations. The collector module is easily
removed to facilitate cleaning and maintenance
and to ensure fast, easy color change.
60
Powder Spray Booths and Recovery Systems
POWDER
SPRAY GUN
HIGH-VOLTAGE
POWER SUPPLY
r
SPRAY BOOTH
lORKPlECE
OTARY SIEVE
CARTRIDGE FILTER
COLLECTOR WITH
COLOR MODULE
FEED HOPPER
1
AIRFOIL FAN
FINAL
RUM UNLOADER
flRGlN POWDER)
Figure 6-8. The efficient side draft spray booth delivers high material utilization and fast color change.
Open Collector Cartridge Filtration
feature ensures easy access to facilitate cleaning
and color changing operations.
This is the most common and efficient method
of overspray recovery in powder coating applications. Open collector refers to the open-face
design of the collection unit (Figure 6-9) that
eliminates the need for explosion venting. Powder is separated from an air stream, drawn through
a cylindrical filter element. The powder is then
removed from the filter element by a reverse air
A level sensing device in the external feed
hopper automatically controls powder flow of
recovered overspray from the collector back to
the feed hopper on demand. This improves
fluidization of powder material and ensures
optimum coating performance.
61
Chapter 6
\
+
7
V
%
?
Figure 6-9. The open collector is so named because of its open-face design, which eliminates the need for
explosion venting.
jet pulse mechanism. The powder ejected from
the surface of the filter falls into a collection zone
and is then processed for return to the spray
equipment. The air drawn through the cartridge
filters passes through a secondary high-efficiency
filter, to remove any fine powder particles before
the air is returned to the work environment.
Efficiency of the cartridge filtration is a function
of several key design elements.
Depending on the quality of the design, some
portion of a tightly pleated surface area may not
be accessible in actual operation. Some filters are
enclosed in a perforated metal screen to protect
delicate paper media. This, however, can also
reduce the effective surface area. With new highstrength polyester media, an open pleat design
and the elimination of an outer screen are now
feasible design elements (Figure 6-10).
For most efficient cleaning with minimal
waste, the filter cartridge should be designed for
maximum efective surface area. The traditional
air-to-cloth ratio associated with bag filters is not
an appropriate measure for pleated filter elements.
Filter efficiency, as measured for a given
powder particle size, and static pressure drop
across a filter are the two important measures of
potential filter performance. Powder formulation
and media composition must also be considered.
62
Powder Spray Booths and Recovery Systems
/
-
.-
Figure 6-10. Filter cartridges are becoming more effective because of open pleat design and the elimination of outer
screens. Making such innovations possible is the new high-strengthpolyester media.
Reverse-jet Cleaning Mechanism
powder-laden air throughout the filter element
population. This is important to assure minimal
disruption of the spray region and full utilization
of filter media.
A filter cartridge cleans as a function of the
rate of pressure build-up inside a cartridge
element. This build-up is induced by a jet of
compressed air that fills the cartridge interior
while sealing the cartridge opening. A welldesigned cleaning mechanism is essential to the
correct operation of a filter cartridge collection
system.
Cartridge Mounting Arrangement
The best design for mounting filters minimizes
the number of surfaces to seal and maximizes
utilization of the filter media. The most common
cartridge arrangement is vertically hung filters,
either single or double stacked, in a side-draft
module (Figure 6-11). This arrangement minimizes re-entrapment of powder on collateral
filters following pulse cleaning. The side location
Collector Inlet Design
Appropriate inlet design includes baffling for
turbulence reduction and distribution of the
63
Chapter 6
Figure 6-1 1. The most common cartridge arrangement consists of vertically hung filter, either single or double
stacked, in a side-draft module.
helps reduce the height requirement of the booth
floor. Cartridges can also be mounted horizontally (usually in stacks of two), either below the
spray area in a down-draft arrangement or in a
side-draft module (Figure 6-12). When filters are
mounted horizontally in the side-draft module,
sometimes the filter stacks are staggered to reduce
the tendency for powder to be recaptured on a
lower set of filters once released from an upper
filter stack.
COLOR CHANGE
CONSIDERATIONS
Color change in a powder system requires
certain activities common to all types of equipment. Guns should be blown out using compressed air and, in some cases, require
disassembly. Feed hose should be blown out or,
in extreme color changes, replaced. Pumps or
injector blocks should be cleaned with compressed air. Booth walls must be cleaned, typically with a squeegee, by carefully wiping them
down with the booth exhaust air in operation.
Color change speed may depend upon the versatility of the reclaim system. Duplicate hoppers
and sieves may be necessary to complete quick
color changes. When time is not a factor or
multiple booths are used, solitary reclaims are
satisfactory, but all areas contacting the powder
must be vacuumed or wiped.
One former major drawback to powder coating
was the time required to change colors. However,
equipment suppliers have made various design
improvements to all aspects of the powder coating
system to minimize the color change time. One
approach is a duplicate powder coating line.
When it is time to change colors, the system
containing the current color is rolled off line,
Powder Spray Booths and Recovery Systems
POWDER SPRAY
4
SPRAY BOOTH
ROTARY SIEVE
FEED HOPPER
H
P
FINAL FILTERS
COLLECTOR
Figure 6-12. Cartridges can be mounted horizontally either below the spray area in a downdraft arrangement or in a
side-draft module.
the various powder colors. Caution should be
exercised if compressed air is used to clean the
booth interior. Air-blown powder may overcome
the booth face velocity and migrate outside the
spray booth. A rubber squeegee or damp cloth
may be more desirable for the booth cleandown.
while the system with the next color is positioned
on line. This allows the system in the off-line
position to be cleaned in anticipation of any
additional colors. The number of duplicate
powder coating systems required depends on how
many colors are sprayed and how frequently the
color changes take place.
Finally, the recovery system must be prepared
for the next color. The procedure for preparing
the recovery system varies, depending on the style
of the system. Equipment manufacturers have
made design improvements in all areas of the
powder coating system. Color change within a
single powder coating system can be accomplished in minutes.
When color changing a single powder coating
system, three areas require attention. First, the
application equipment (generally consisting of the
spray guns, powder hoses, injectors, and feed
hopper) needs to be either cleaned or changed.
Second, cleaning down the spray booth interior
is necessary to prevent any cross contamination of
65
CHAPTER 7
DESIGN CONSIDERATIONS FOR
OPERATION OF POWDER COATING
SYSTEMS
Insurance ratings: the materials of construction can have a bearing on insurance rates for
the building, and this could lead to a substantial saving on premium dollars.
Having the proper facilities is one of the most
important aspects of a powder coating system for
producing a consistently high-quality product.
Being able to design the facility from the ground
up is the ideal situation. Yet many systems must
be installed within the available plant space.
Surface smoothness: smooth interior walls
lend themselves to easier maintenance,
thereby saving labor.
In every locality, guidelines and parameters
exist for air quality and liquid and solid waste
disposal. These should be carefully reviewed
with local regulatory agencies prior to designing
the powder coating system. Such discussions
may also have a bearing on site selection.
Migrating powder from a system (though not
usually a problem in a well-designed system) can
occasionally become troublesome. A small
amount of powder covers a large area if it is
picked up by stray air currents. It accumulates
over time on ledges and structural members.
Enclosures with smooth walls and no catchall
framework are desired for easy and best maintenance.
Factors inside and outside your facility make
general location of the operation important. For
example, even a properly designed powder
coating booth can have failures that result in
accidental migration of powder to the surroundings. Thus consideration should be given to
ambient in-plant operations that could be affected
by such a failure.
When making an equipment layout for a
powder operation, the location of all elements
should be carefully analyzed. Overhead framing
can be avoided at this stage. It is best if exhaust
from booths and ovens is direct. Conveyors are
most easily mounted when positioned relative to
adequate building structural members.
Outside, consider if any neighbors’ exhausts
will be detrimental to the powder coating processing and vice versa. A lumber mill may not be the
best neighbor for a powder coater, but probably
would be better than a cement plant. The same
concept applies to the location of equipment
within the plant.
The relationships between the framing and
exhaust from equipment are often overlooked.
While some buildings have sufficient load factors
built into the framing to support conveyors, others
may not. Investigate the load factors before
suspending anything from the framing.
The construction materials for an environmentally controlled room should be looked at in the
following areas:
The location of windows and doors in relation
to spray booths and ovens is of great importance.
Spray booths are designed to keep the powder
within the booth and to provide uniform airflow
Insulating properties: depending on the
geographic location of the facility, the
insulating properties of the construction
materials could save a great deal of energy.
67
Chapter 7
as needed to transport the powder. A door or
window opened next to, or in the vicinity of, a
booth may completely disturb the booth airflow
and cause difficulty. Powders may leave the
booth, distorting the spray pattern, and drawing
outside dusts onto the coating. Check with the
equipment supplier on these points.
powder will land on the part, resulting in either
rejection or rework. Also, the contaminant will be
distributed throughout the system, contaminating
otherwise clean material. Another consideration is
that the contaminant could cause blockage, or
"blind" the filter media, reducing the airflow
through the system. Substantial reduction of
airflow creates an unhealthy and unsafe atmosphere; powder containment capacity within the
booth is reduced and powder could drift out into
the work area. Also, as airflow decreases through
the spray booth, the minimum explosion concentration of the powder being sprayed could exceed
safe limits. (Most systems are designed with
interlocks that disengage or shut the system down
when airflow decreases to an unsafe or unhealthy
level.)
Although it is not always attainable, a controlled atmosphere for the coating booth and
powder storage is very desirable. In some areas,
this is necessary because of ambient conditions of
temperature and humidity. Powder, if exposed,
readily picks up moisture and is generally somewhat temperature sensitive. Operation will be
improved if conditions can be standardized.
Check with your powder supplier for special
considerations.
Gages are normally provided with most
powder coating systems. They indicate pressures
across the face of the filter media and in the clean
air plenum of the filtered air retuming to the work
area. Gages should be checked daily and, if
possible, logged to be used as references for the
safe operation of the system. In essence, air
provided for the system should be clean, dry, and
oil-free. Refrigerated air dryers will remove
moisture from the air and should be included in
any powder installation. Oil removal filters
should be installed in the main air supply line to
the system and checked or drained daily.
Adequate consideration should be given to the
availability and accessibility of all utilities.
Energy is needed in a powder system to heat
solutions and to cure the applied powder. Water is
needed in the metal preparation stages. Disposal
facilities must economically handle spent solutions and solids residual to the system. Every
advantage should be taken of consultations
offered by local utility and sanitation people. Be
certain that you are aware of their services and
they are aware of your needs.
OPERATION OF POWDER
COATING SYSTEMS
Another point to consider in maintaining
cleanliness is the delivery and recovery equipment involved. To provide maximum efficiency,
guns, pumps, hoses, feed hoppers, distribution
hoppers, and any surface powder may encounter
should be checked for cleanliness. Lumps of
powder, impact fusion, debris, and wom powder
contact parts block powder flow. Properly cleaned
powder contact parts will prove most beneficial in
controlling powder flow to and from the spray
guns, and will reduce the frequency of cleaning.
Parts not properly cleaned increase impact fusing
and wear of parts. One note of caution: never
clean powder contact parts with sharp or abrasive
tools. A nick or scratch greatly enhances powder
buildup on the part.
Components of a powder coating system
obviously rely on each other to maintain a safe
and healthy operation. Proper operation of each
component will ensure a safe and healthy working
environment, and will result in maximum productivity. The following factors should be considered
in operating and maintaining a power coating
system.
System Cleanliness
Powder introduced into the system should be as
clean as possible, free of debris, moisture, oil, or
other contaminants. Any contaminant in the
68
Design Considerations
to justify operation are easily attained. Some hints
for maintaining system efficiencies are:
It is also very important to provide clean parts,
hooks, racks, and conveyor. This enhances the
electrostatic effect by ensuring a properly
grounded part, and helps ensure a safe operation.
0
Operation of System Components
Check oil and dirt-removal filters installed in
air lines on a regular basis. Drain filters often
and replace when necessary.
Check operation of refrigerated air dryer at
least once a week. Just because the air dryer
is running, does not mean it is operating
properly. Gages will indicate inlet and outlet
temperatures of compressed air.
Proper design and construction are the first
steps toward a successful and profitable powder
coating system. Each powder coating system is
unique in both design and application. Specifications the user provides to the powder equipment
supplier determine the parameters of the system
design. Information should always include all part
dimensions, line speed, film-build requirements,
types of hooks or racks to be used, conveyor
height, dimensions of space provided for system
installation, production schedule based on batch
sizes and number of colors involved, and projected production requirements for the future.
This enables the equipment supplier to design a
system to exact specifications, for many years of
utilization.
* Check powder contact parts periodically for
wear. If the parts show excessive wear,
replace them. Failure to do so will result in
more time spent in cleaning the part to
remove impact fusion, and will affect the
system’s operation. Never use sharp objects
to clean parts. Always maintain a reasonable
supply of spare parts.
Check all ground connections on a regular
basis. The loss of ground could affect the
transfer efficiency of the spray guns, and a
hazardous situation could be created if ground
between workpiece and hanger is lost. Strip
hangers on a regular basis.
Information concerning the type of metal
preparation and curing facilities also should be
provided.
During the first few weeks of operation, the
system should be monitored closely to determine
its particular operating characteristics. These
observations can be used to establish operating
procedures for each product conveyed through the
system for powder coating application. Remember, the system design is based on the largest part
that will be coated and finished. Smaller parts
may require adjusting the number of guns used,
powder flow, electrostatic setting, proximity of
the gun to the part, or any combination of these
variances. By knowing in advance the precise
settings and making these adjustments, excessive
powder usage can be avoided and the desired
efficiencies will be achieved.
Check for pressure drops across filter media.
A reduction in airflow through the booth
could result in poor powder containment; an
unsafe situation could be created if the
minimum explosion concentration exceeds
safe limits. Clean absolute filters on a regular
basis. If absolute or final filters require
frequent cleaning, it could indicate a ruptured
or leaking filter media. The media should be
checked and replaced if necessary.
Clean booth daily with a squeegee or other
grounded nonsparking device. Never use rags
or paper towels to clean within a booth area.
Lint could contaminate the powder material
and create finish problems. More extensive
cleaning should be accomplished on a weekly
or bi-weekly basis.
A solid maintenance procedure based on daily,
weekly, and monthly system preventive and
corrective maintenance measures should be
initiated. If this is done, the efficiencies necessary
69
Chapter 7
maintenance on equipment, andlor disposing
of empty material containers. Powder, when
exposed to the skin for extended periods of
times, can dry the skin.
Clean powder spray guns and feed pumps
daily. Use an air hose to blow powder from
the hoses, pumps, and guns. Visual inspection
will reveal the need for additional cleaning or
parts replacement. Be sure there are no loose
hose connections between pumps and guns.
All air line fittings should be secure. Locate
feed hoppers as close to guns as possible to
reduce hose length and the possibility for
kinks or bends.
Facilities should be provided for proper
washing of skin exposed to powder materials.
Soap and water are considered to be the safest
and most effective means of cleansing skin
exposed to powder. Personnel should be
encouraged to wash with soap and water
frequently. It is certainly necessary before
eating, drinking, or performing normal body
functions. Skin reactions can occur in some
cases when exposed to powder and should be
treated by frequent washing. Cleansing the
skin with organic solvents should never be
encouraged.
Post maintenance procedures in areas where
operators and supervisors will see them.
Provide check-off lists for maintenance
procedures.
Operational Health Hazards
to Operators
Exposure to respiratory inhalation should be
prevented through the use of respirators or
masks. Also, proper ventilation of the powder
spray system is important to prevent exposure
to respiratory inhalation. Proper ventilation
also maintains an environment safe from
explosions by minimizing the possibility of
ignition from static electrical sparks or other
ignition sources. (NFPA 33, Section 13-5
specifies and establishes proper ventilation
guidelines.) The safest operating procedures
specified for powder spray applications are
also the most productive.
Most powder coating materials contain a
variety of substances to formulate the ultimate
coating material. Some may pose health hazards
to personnel within the immediate spray area.
Pigments, curing agents, polymers, and fillers all
present potential health hazards if permitted to
escape the spray containment area. Such hazards
can be caused by improper ventilation or-by
improper handling or use of the powder. OSHA
regulations, applying to user and supplier, govern
both the handling and use of powder coating
materials. A Material Safety Data Sheet must be
provided by the supplier, advising the user of any
hazards associated with the powder coating
material. Recommended precautions concerning
skin contamination and respiratory exposure are
normally documented in the Materials Safety
Data Sheet.
The electrostatic powder coating process is
somewhat different from the more conventional
means of applying product finishes (such as “wet
spray”). This is a method of applying electrically
charged powder material to grounded parts.
Powder is held to this part by the electrostatic
charge imparted to the powder during the spray
process. To ensure attraction and transfer efficiency, two conditions must be met. First, the part
being sprayed must be properly grounded. Second, the electrostatic spray equipment must be
functioning properly. If either of these conditions
is not met, the net result is poor transfer efficiency
and, in the case of a poorly grounded part, ultimately an unsafe condition.
The following recommendations should be
considered to reduce potential health hazards
associated with powder coating materials.
Gloves and dust masks should be worn by
personnel involved in handling the powder
material. They are needed when opening fresh
material containers, dumping material into
supply hoppers, cleaning or performing
70
Design Considerations
Mutual Engineering Corp. (FM), Edison Test
Labs (ETL) and others in the United States. In
Canada, certification by the Canadian Standards
Association (CSA) is required. Approved status
of the equipment means the equipment has been
tested and evaluated by a nationally recognized
approval agency and has met the established
standards of safety.
Safety should always be incorporated into
operating and maintenance procedures for the
powder coating system. These procedures should
consider all aspects of operation including:
Storing and handling powder materials;
Spraying parts within the spray booth;
Conveying parts through the spray booth;
Spray operations must be confined to properly
designed spray booth, spray rooms, or designated
spray area.
Cleaning and maintaining equipment;
Troubleshooting equipment;
All spray areas should be provided with
mechanical ventilation adequate to transplant
flammable or combustible dusts, vapors, mists,
residues, or deposits to a safe location. Ventilation for spray booth should be adequate to confine
air suspended powder to the booth and recovery
system at all times. Average air velocity through
electrostatic booth openings shall not be less then
60 ft. (18.2m) per minute (however, many sources
say the velocity should be at least 90 ft. (27.7m)
per minute).
System start-up and shutdown;
Reading, calibrating, and setting control
gages and regulators;
Recording daily critical ventilation pressure
readings;
* Responding to alarms, interlocks, and system
safety oriented control devices; and
Parts being coated should be supported on
conveyors or hangers properly connected to
ground, with a resistance of 1 megohm or less.
Disposing of waste materials
Guidelines for proper safe operating procedures are provided by the National Fire Protection
Association (NFPA) in several publications and
have been incorporated into governing OSHA
regulations. Most important, reference guidelines
can be found in:
All electrically conductive objects in the spray
area, except those objects required by the process
to be at high voltage, should be adequately
grounded.
Spray areas must be protected with an approved automatic fire extinguishing system.
NFPA Bulletin 33;
OSHA Section 1910.107; and
Fixed powder application equipment should be
protected further by approved flame detection
apparatus that will, in the event of ignition, react
to the presence of a flame within one-half second
and:
NFPA Industrial Fire Hazards Handbook,
Chapter 25.
Here are some highlights from these references.
Shut down all energy supplies (electrical and
compressed air) to conveyor, ventilation,
application, transfer, and powder-collection
equipment;
First, equipment should be listed and approved
by nationally recognized approval agencies, such
as Underwriter Laboratories Inc. (UL), Factory
71
Chapter 7
Close segregation dampers in
associatedductwork to interrupt airflows from
application equipment to powder collectors;
and
These highlighted guidelines are obviously
directed more toward the safe operation of a
powder coating system than toward a productive
operation. However, as stated before, the safest
operations are also generally the most productive
operating powder spray systems.
Activate an alarm.
72
HEATING APPLICATION
Heating operations are required for a variety of
purposes on a powder coating line. These are
usually accomplished with ovens of various
forms, selected according to the needs of a
particular operation. They can heat parts, such as
porous castings, to rid them of occluded gas; burn
accumulated material from part supports in a
cleaning step; raise the temperature of previously
cleaned parts to prepare them to accept powders,
as in the fluidized bed process; and reheat coated
and cooled parts to promote further flow of the
applied material. Also, heating can remove water
remaining from prior operations that would cause
trouble if carried to the powder coating station.
Most important, heating is used to melt, flow, and
cure powdered materials applied at room temperature by electrostatic spray processes.
function. Temperatures-at most-can be in the
range of 190°F (88°C) and can be attained with
any number of available ovens. Ventilating the
area is of little significance. Water vapor must be
removed to allow the time-dryness relationship.
Also, preheating is needed to raise the temperature of parts to be coated by processes that “fuse”
the powder on the part. This requires the establishment of a temperature-time coating cycle so
coating thickness can be reproduced. The hotter
the part, the more the amount of material adheres
before the temperature falls below the fusion
point. Heat can reduce the Faraday Cage effect by
allowing penetration into crevices hard to penetrate otherwise. The temperature-time curve for
such ovens must be known and reproducible.
Generally, such ovens need not be ventilated
beyond required safety considerations. These
conditions can be met easily by any number of
commercial ovens.
Since powder coating operations are usually
conducted at high speed under a production
environment, heating functions must be carried
out in the most efficient and cost-effective
manner. Requirements of a particular application
must be thoroughly studied and matched with the
capabilities and design principles of available
ovens. Thoroughly investigating all aspects of the
heating components of a powder finishing line is
critical to achieving an efficient, effective, and
satisfactory operation that will produce a highquality product.
POSTHEATING
Postheating operations on a powder finishing
line are perhaps the most critical. They are used to
melt, flow, and cure the powder applied to the
part at ambient temperature, as with the electrostatic spraying process. This application of heat
has to be very carefully controlled because
temperature-fluidity characteristics of a particular
powder are peculiar to that powder. It is this
relationship that determines how the flow of the
material will take place as the temperature is
raised. Most materials cross link and become
more viscous with time at a given temperature.
For thermosetting materials, this process further
complicates the control.
PREHEATING
Parts on their way to the coating application
are preheated when it is necessary to “dry off’
any moisture remaining from previous cleaning or
conditioning steps. Such heating must be only
adequate enough to accomplish this simple
73
Chapter 8
ovens utilize electric low-intensity infrared
elements (calrods, resistance emitters) to provide
a clean, safe method of convection heating.
Final properties of the coating can be acquired
only uniformly over the part if all areas of the
coating are treated in thermally equivalent
conditions. For this reason, the postheat ovens
again must be of high quality and equipped with
adequate controls to ensure reproducibility.
The time required to bring powder deposited
on the part to its cure temperature largely depends
on the mass of the part and the rate at which the
part accepts heat using convection heating. Large
metal objects may require 30 minutes or more to
reach the desired cure temperature. Smaller parts
can be brought to temperature much more rapi d l y 4 to 8 minutes. This heat absorption by the
part, other than the surface, is a waste of heat
energy as far as powder curing is concerned. But
considering process requirements, such as part
configuration, products with varying size and
dimension and processes, where products of
similar configuration cannot be batched, this may
be the most flexible and efficient method of
curing.
PROCESS CONSIDERATIONS
Many elements are variables that must be
considered and studied when selecting ovens for
the powder coating line. Of utmost importance
are:
Product: size, configuration, mass, temperature limitations.
e
0
Conveyor: method, product holder, line
speeds.
Powder: formulation type, thickness, cure
profile, color, gloss, test for cure.
Temperature response time of large convection
ovens is not particularly rapid, often taking an
hour or more to reach operating temperature from
a cold start. Convection ovens require consideration of floor space and routine cleaning if good
finishes at high production rates are to be obtained. Despite these limitations, convection
ovens are currently the most popular for industrial
painting and curing. Energy and operating costs,
however, are often high, and users should be
aware of alternative methods for their production
lines.
All parameters should be defined and communicated to equipment and powder suppliers
involved to ensure an integrated system and
successful installation.
AVAILABLE METHODS
OF HEATING
Several individual methods and combinations
of heating techniques have been successfully used
to cure various powder coatings in a wide range
of applications. New technology in curing ovens
and powder coatings have allowed dramatic
improvements in cure results in the last few years.
Infrared Radiation
Short wave, high-intensity infrared heating
uses electrical energy to produce a direct, radiant
method of heating. Infrared is transmitted directly
from emitter to product via electromagnetic
waves traveling at a speed of light (186,000
miles/second). Unlike convection heating, high
intensity infrared requires no medium for heat
transfer. Radiation is a line-of-sight method; it
only cures what it sees. Heated energy is transferred quickly, cleanly, and efficiently, typically
with tungsten quartz infrared lamps. Short-wave
heaters also penetrate the substrate. High intensity
infrared can have fast temperature-time response.
Hot Air, Gas, or Electric Convection
Convection heating uses air as a medium to
transfer heat from the energy source to the
product. Many convection systems use a fuel
source (gas, oil, or steam) that provides heated air
circulation in the oven chamber. Using a combustion chamber, the oven atmosphere can contain
combustion products, solvent vapors, and possibly traces of unburned fuel. Other convection
74
Heating Application
Curing ovens using this method of radiation
heating are compact in size and can be zoned to
match exact product configuration and size. Oven
start-up times of 10 to 15 minutes are common.
Savings in energy, space, and time can be realized
with high-intensity infrared if the part configuration is correct for infrared heating.
in all powder curing applications is routed outside
the building. Powders contain no hazardous
solvents. The exhaust and supply air systems are
arranged to provide a slightly negative pressure
within the oven. Large convection ovens are often
located near factory roofs because of the heat loss
and flow to the work area around the oven. Highintensity infrared systems do not provide a heated
area around the oven, however. Ovens must be
sized to permit adequate cure of the powder and
air flow.
Infrared radiation is best used with products of
consistent shape, produced on dedicated lines
with large volume. A direct line of sight-heater
to object surface-is needed for proper powder
cure, and cure is affected by object distance.
Rotating the coated product during the infrared
cure for round or cylindrical shape product
provides uniform heating with consistent cure
results. Reflow and cure times range from 10
seconds to 120 seconds, using high intensity
infrared.
Oven cavity wall construction is often steel or
ceramic. Walls are typically assembled from
modules and are insulated to contain heat within
the oven.
SAFETY
The exhaust fan is a primary oven safety
feature. It removes solvents and combustion
vapors to avoid explosion hazard. Powder vapors
must be removed to keep the color from discoloring from the materials that come off the powder
during the curing process. Exhaust and recirculation fans must be equipped with air flow sensors.
These devices will monitor serious disruptions in
air flow, and shut down the oven if any are
detected.
OVEN CAVITY ATMOSPHERE
Oven cavity atmosphere is critical to both
safety and product appearance. Solvent or vapor
concentration must be kept to not more than 25%
of the lower explosion limit (L.E.L.). Thus, a
specific amount of oven air may be exhausted and
replaced on a continuous basis during operation.
Fresh input air should be introduced to the oven
for solvent or vapor dilution, and then exhausted.
In powder applications, the amount of solvent
released is near zero. Volatiles may consist of
low-molecular weight resins, water, blocking
agents, etc., but usually no solvents. Exhausted air
Manufacturing safety provisions and regulatory
requirements should be followed for safe operation. All oven safety features should be checked
on a quarterly basis.
75
CHAPTER 9
POWDER STORAGE
AND HANDLING
Powder, like any coating material must be
shipped, inventoried, and handled in its journey
from the powder coating manufacturer to the
point of application. Manufacturers’ recommendations, procedures, and cautions should be
followed. Although various powders may have
specific requirements, some universal rules apply.
It is important that powders should always be:
Manufacturers recommend long-term storage
temperatures of 80°F (27’C) or lower.
Unless its exposure to heat has been excessive
over an extended period of time, powder that has
experienced such changes can usually be broken
up and rejuvenated after being passed through a
screening device.
Powders with very fast or low-temperature
curing mechanisms may undergo a chemical
change as a result of exposure to excess heat.
These powders may partially react or “B stage.”
Even though these powders may be broken up,
they will not produce the same flow and appearance characteristics as unexposed powders. They
will have, and irreversibly retain, restricted flow,
even to the point of a dry texture.
Protected from excess heat;
Protected from humidity and water; and
e
Protected from contamination with foreign
materials, such as other powders, dust, dirt,
etc.
These are so important, they deserve more
elaborate explanations.
Powders formulated with chemical blocking
agents to prevent curing below certain trigger
temperatures do not typically “B stage” at temperatures below 200°F (93°C).
Excess Heat
Powders must maintain their particle size to
allow handling and application. Most thermosetting powders are formulated to withstand a certain
amount of exposure to heat in transit and in
storage. This will vary according to types and
formulation, but can be estimated at 100-120°F
(38-49°C) for short-term exposure. When these
critical temperatures are exceeded for any length
of time, one or all of the following physical
changes may happen. The powder can sinter,
pack, andor clump in the container. Pressure of
powder weighing on itself (Le., large tall containers) can accelerate packing and clumping of the
powder toward the bottom of the container.
Protect from Humidity and Water
Water and powder do not mix when the intent
is to spray as a dry powder. Exposure to excessive
humidity can cause the powder to absorb either
surface or bulk moisture. This causes poor
handling, such as poor fluidization or poor gun
feeding, which can lead to gun spitting and
eventually feed hose blockage. High moisture
content will certainly cause erratic electrostatic
77
Chapter 9
5. Maximize powder transfer efficiency in the
booth to avoid problems associated with
recycling large quantities of powder.
behavior, which can result in changed or reduced
transfer efficiency and, in extreme conditions,
affect the appearance and performance of the
baked coating film.
6.Minimize the amount of powder coating
material held on the shop floor if temperature
and humidity of application areas are not
controlled.
Contamination
Because powder coating is a dry coating
process, contamination by dust or other powders
cannot be removed by filtering, as in liquid paint.
It is imperative, therefore, that all containers are
closed and protected from plant grinding dusts,
aerosol sprays, etc.
SAFETY
Powder coatings contain polymers, curing
agents, pigments, and fillers that require safe
operator handling procedures and conditions.
Pigments may contain heavy metals, such as lead,
mercury, cadmium, and chromium. The handling
of materials containing such elements is controlled by OSHA regulations. End use may be
restricted according to Consumer Product Safety
Commission Regulations.
STORAGE RECOMMENDATIONS
Storage stability properties of powder coatings
need not cause problems at the end user’s facility,
provided that a few simple precautions are taken.
Among these precautions are:
Under some circumstances, OSHA regulations
require the applicator to inform employees of the
hazards associated with handling certain components or powder coatings. The applicator is
advised to obtain this information from the
supplier in the form of a Material Safety Data
Sheet. Powder coatings should be handled in a
manner as to minimize both skin contact and
respiratory exposure consistent with particular
Material Safety Data Sheet recommendations.
Obvious health reactions attributed to any powder
coating operation should be referred to a physician as soon as possible.
1. Control temperature, 80°F(27°C)or less.
Remember that powder requires minimal
storage space. For example, a semi-tractortrailer-sized area can accommodate 40,000
lbs. (18,143 kg) of powder, which is approximately equal to 15,000 gallons (56,775L)of
liquid paint at applications solids.
2. Efficiently rotate the stored powder to
minimize inventory time. Powder should
never be stored for a period exceeding the
manufacturer’s recommendation.
Opening, emptying, and handling powder
containers, such as boxes and bags, often present
the greatest worker exposure, even with welldesigned systems. Engineering practices, personal
protective equipment, and good personal hygiene
should be used to limit exposure. In a welldesigned spray operation, there should be negligible exposure of employees to dust. Powder
coatings, because of their fine particle size and
frequently large percentage of TiO,, will absorb
moisture and oil readily.
3. Avoid having open packages of powder on
the shop floor to preclude possible moisture
absorption and contamination.
4. Precondition powder prior to spray application by providing preconditioning fluidization, as is available on some automatic
systems, or by adding virgin powder through
reclaim system. These techniques will break
up the powder if minor agglomeration has
occurred in the package.
78
Powder Storage and Handling
If powder is left in contact with the skin for
extended periods, it tends to dry out the skin. To
should be
prevent this, gloves and clean
worn by the workers. Operators of manual
electrostatic guns must be grounded. To prevent
carrying powder away from work, workers should
change clothes prior to leaving the workplace. If
powder does get on the skin, it should be washed
off at the earliest convenient time, at least by the
end of the day. Workers who show skin reactions
on exposure to powder must be especially careful
to wash frequently. Washing the sfin with organic
solvents is an unsafe practice that should be
forbidden. Generally, cleansing with soap and
water is the appropriate hygienic practice. Additional information should be obtained from the
supplier’s Material Safety Data Sheet.
79
CHAPTER 10
REPAIR OF PARTS AND HANGER
STRIPPING
Touch-up should not be used to repair a faulty
finish unless the resulting product meets inspection standards.
The methods of part repair after powder
coating can be put into two categories-touch-up
and recoat.
RECOAT
Touch-up repair is appropriate when a small
area of the coated part is not covered and is
unable to meet finishing specifications. When
hanger marks are not acceptable, touch-up is
required. Touch-up also may be used to repair
slight damage from handling, machining, or
welding during assembly.
Applying a second coat of powder is the
common approach to repair and reclaim rejected
parts. However, the defect should be carefully
analyzed and the source corrected before
recoating. Do not recoat if the reject is caused
from a fabrication defect, poor quality substrate,
poor cleaning or pretreatment, or when the
thickness of two coats together will be out of
tolerance. Also, if the part is rejected due to
undercure, it merely needs to be rebaked at the
required schedule.
Recoat is required when a part is rejected
because of a large surface area defect or when
touch-up is not acceptable. At this point, there is a
variety of options that should be capsidered
carefully. Usually the rejected part can be repaired with a second coat. Another option is
stripping and repainting the part. Stripping can
also clean part hangers to provide a good ground
for electrostatic spray.
A second coat is effective to cover light areas,
surface defects from dirt and contamination,
rough spots from heavy film build or gun spitting,
ind color change from severe overbake. Rough
surfaces and protrusions should be sanded smooth
before recoating.
TOUCH-UP
Liquid touch-up paint is applied with a small
brush, aerosol spray, or airless gun. The paint is
air-dried. The drying process can be accelerated
with a low-temperature bake. Touch-up paint is
used after the powder coating has been fully cured
in a bake oven. Hanger marks, light spots in
comers and seams, damage from welding or
assembly, and other small defects can be touched
up. Generally, a color-matched acrylic enamel or
lacquer is used. Touch-up paint cannot be used if
it will not meet the performance specifications
required during the expected life of that part.
Parts inspected on-line can be left on the
conveyor to receive a second coat. These parts
can pass through the pretreatment stages with raw
parts. If the recoated parts show water spots or
stains, an adjustment can be made in the final
rinse stage. Chemical suppliers can offer recommendations. When parts for recoat are hung
together, cleaning and pretreatment is not necessary. However, if the rejected parts have been
stored to accumulate a practical number, they
should be checked for dirt and contamination.
81
Chapter 10
Coat Entire Part
Chemical strippers are available to be used hot
(raised temperature) or cold (ambient) in a dip
tank. There are acid, alkaline, and molten salt
types, with selection depending on the type of
parts and hangers and the coating to be removed.
The main advantage of chemical strippers is the
low initial capital investment for equipment.
Disadvantages include safety hazards of handling
the chemicals, high costs of chemical replacement
and disposal, and chemicals laden with paint.
Some parts, such as aluminum alloys, may not be
able to withstand corrosion of the chemicals.
When applying the second coat, normal mil
thickness should be applied to the entire part. A
common mistake is to coat only the defect area.
This leaves a rough gritty surface where there is
only a very thin overspray layer on the remainder
of the part. The same recommended cure schedule
is used for the second coat.
Intercoat adhesion can be checked after
recoating on selected samples by using the cross
hatch test or simply scratching the surface to see
if the second coat peels easily from the first.
Some powder coatings may need to be lightly
sanded to provide a good anchor for the second
coat.
Burn off
Burn off, or pyrolysis, ovens for stripping use
high temperatures to incinerate the coating. They
can be batch type or on-line ovens that operate at
about 800°F(427"C), with the pollution control
exhaust operating at temperatures of approximately 1200-1300°F(649-704°C). Burn-off ovens
eliminate pollution and disposal problems. They
are relatively efficient to operate, but require large
capital investment and need some type of post
cleaning to remove residual ash. The parts must
withstand 800°F (427°C) temperatures. Some
coating chemistries are not suitable for this
stripping technique. Consult the equipment
manufacturer and local regulatory agencies. It
should also be noted that repeated stripping of
tooling may require some type of alloy to prevent
breaking or deforming.
REBAKE
When a part is undercured during the first coat,
it can be repaired by just returning it to the bake
oven for normal cure schedule at the specified
time and temperature. Properties will be recovered when the part is properly cured, with some
exceptions, such as certain chemically controlled
low-gloss coatings. Partial cure will result in a
higher gloss, which does not drop to the same
level during final cure that would have been
obtained with an adequate initial cure.
STRIPPING
Shot Blasting
Stripping is usually the last alternative for part
repair since stripping rejected product can add
greatly to production cost and disrupt the production line flow. Stripping coated parts becomes
necessary, however, when the reject is caused by
poor pretreatment or when touch-up or two coats
are not acceptable.
Shot blasting, or abrading, can be used to strip
parts or hangers when other methods have been
ruled out. This process is very slow due to
toughness of the cured powder coating. The
disadvantage of this process is that it erodes
(thins) the tooling and exposes more surface area,
which becomes harder to strip when recoated.
On the other hand, stripping plays an important
role in effectiveness of the powder coating line by
providing clean hangers for a good electrical
ground. Hangers should be stripped periodically.
Stripping methods are discussed in the following
paragraphs. (Note: There is a difference of
opinion that chemical stripping is the preferred
method.)
Cryogenic
Cryogenic stripping embrittles the film with
liquid nitrogen, then uses a nonabrasive shot blast
to easily remove the coating. This is a fast,
nonpolluting method, but it requires specialized
82
Repair of Parts and Hanger Stripping
equipment. Parts must endure -100°F (-37°C)
temperatures, and some type of alloy may have to
be considered for tooling.
described. Chemical and equipment suppliers can
assist in that determination.
When it comes to tooling, proper design can
reduce the amount of cleaning necessary. An
inexpensive part hook can become very costly if it
must be replaced frequently.
GENERAL
Consideration must be given as to whether or
not parts can withstand any of the methods
83
CHAPTER 11
QUALITY CONTROL
Quality control in the finishing industry
requires attention to more than just coating. In
fact, the majority of problems occur for reasons
other than coating faults. To assure quality where
coating may be a factor, statistical process control
(SPC) can be a useful tool.
Since SPC is a data-driven, analytical process,
the numbers themselves must be reliable, with as
little variation as possible. The more variance in a
reading, the wider the SPC control chart limits are
for that variable and the less sensitive it becomes
to changes in the process.
SPC
Formal experiments reveal the capability of
your measurement system for the parameter of
interest. These include tests such as gage R&R
studies and short term machine capability studies.
Readily available is literature on how these
studies are performed.
SPC involves measuring the powder coating
process using statistical methods and improving it
to reduce variation at desired process levels. SPC
can also help determine the difference between
typical variation inherent in the process and
special causes of variation that can be detected
and eliminated.
A quality assurance/quality control system
using SPC enables the powder coating user to be
proactive in preventing defects. It allows decisions to be based upon data rather than on subjective opinions. By using SPC to monitor and
improve critical components in the coating
process, the quality of the final product will
consistently improve, lowering total cost.
A good initial step is to create a process flow
diagram of the system. Be sure to go out on the
shop floor and observe how the process is actually
performed instead of relying totally on how
supervisors and process engineers think it performs.
AVOIDING AND CORRECTING
QUALITY VARIATIONS
Reading the key control characteristics (KCCs)
at each step of the process can then be derived
from the flow chart. These key control characteristics are variables that are most important and
can be monitored using SPC charts.
Close attention to a few critical areas will
avoid, or at least minimize, a multitude of quality
variations with a powder finishing system.
Careful attention should be given to having a
clean, dry, compressed air supply, clean-sieved
reclaim powder, good ground to parts and equipment, humidity-controlled spray booth air, and
regular inspection and replacement of wear parts.
The powder coating equipment should be installed and operated as recommended by the
equipment supplier’s manual. Follow the recommendations on your powder coating material data
A typical list of key variables to monitor may
include:
0
0
Dry film;
Oven cure;
Powder flow rate of virgin and reclaim;
Particle size;
Atomizing air; and
Transfer efficiency.
85
Chapter 11
sheets. Have a good preventive maintenance
program and stringent housekeeping practices.
static coating and presents possible solutions for
six problems.
The following figures provide information on
how to avoid specific variations to quality and
how to correct them if they occur.
Figure 11-9 deals with collection and reclamation operation. Two problems are discussed.
Coating finish-cured film physical properties are
discussed in Figure 11-10 and five problems are
presented. Figure 11-11 relates eight problems
associated with coating finish, cured film appearance.
The first five figures assist the reader in surface
preparation requirements. Figure 11-1 presents a
summary of various application methods. Figure
11-2 is a troubleshooting guide for iron phosphatizing and Figure 11-3 is a similar guide for
zinc phosphatizing. Figure 11-4 presents a
troubleshooting information for the chromium
phosphate processing solutions. Figure 11-5
focuses on the black oxide coating process.
Figure 11-6 discusses fluidize bed operations,
presenting four problems and their various
solutions. Figure 11-7 discusses hoses and pump
venturi operation. Two problems are discussed in
this figure. Figure 11-8 features overall electro-
Figure 11-12 explores the problems associated
with poor output of powder. Three problems are
presented. Figure 11-13 relates problems associated with poor or insufficient coverage. Figure
11-14 presents five problems associated with the
disturbance in cured film. This chapter’s final
figure, Figure 11-15, presents, in graphic form,
suggested temperature and humidity parameters
for applying organic powder coatings.
86
Quality Control
ADVANTAGES
BRUSH- ONNIPE-ON:
No capital investment.
Can handle very large or heavy parts.
Parts do not have to be moved.
Can be startedstopped quickly.
Good for intermittent work schedules.
STEAM GUN:
Low capital investment.
Can handle large or heavy parts.
Parts do not have to be moved.
Can be startedstopped quickly.
Good for intermittent work.
IMMERSION:
Can be automated.
Moderate production rate.
POWER SPRAY WASHER:
Normally automated.
High production rate.
DISADVANTAGES
Labor intensive.
Low production rate.
Labor intensive.
Low production rate.
Moderate capital investment.
Must be able to lift parts.
Requires start-up time.
Higher capital investment.
Requires start-up time.
Figure 11-1. Summary of the various application methods.
87
Chapter 11
PROBLEM
CAUSE
REMEDY
Poor Coating.
pH not in range.
See “Poor Cleaning.”
Adjust pH down with acid, ur
with caustic.
Poor Cleaning.
Temperatures too low.
Concentration too low.
Raise temperature.
Increase concentration.
Poor exposure to cleaner.
Check racking.
Check nozzles.
Check pressure,
15-25 psi.
Spotty Coating/ Streaking.
Contaminated rinses.
Poor cleaning.
Contaminated rinses.
Poor exposure.
Rusting.
Coating weight too low.
Dry-off too slow.
Drying between stages.
Solutions Foaming.
Temperature too low.
Pressure too high.
Pump picking up air.
Poor Paint Adhesion.
Coating too heavy.
Poor cleaning.
Contamination.
Bad steel
Figure 11-2. A troubleshootingguide to iron phosphatizing.
88
Check rinse tanks.
See “Poor
Cleaning.”
Check rinse tanks.
Check racking.
Check nozzles.
Raise temperatures.
Lengthen time.
Increase concentration.
Increase temperature
in final rinse.
Run at lower temps.
Better placement
of nozzles.
Use fog nozzles.
Raise temperature.
Check for plugged
nozzles.
Check pump packing.
Check water level.
Lower temperature.
Lower concentration.
See “Poor Cleaning.”
Look for source of
silicone near washer.
Check raw material
for excessive soil.
Quality Control
PROBLEM
CAUSE
REMEDY
Coating weight too low.
Phosphate or accelerator
concentration too low.
Temperature too low.
Process time too low.
Increase concentration.
Raise temperature.
Lengthen time.
Phosphate or accelerator
concentration too high.
Process time too long.
Decrease concentration.
Powder on coating.
Poor rinses.
Excessive sludge.
Accelerator concentration
too high.
Keep rinse overflowing.
De-sludge tank.
Allow concentration to drop
Spotty coating.
Poor cleaning.
Low concentration of phosphatizer or accelerator.
Poor solution coverage.
Resistant metal.
Check cleaning tank.
Increase concentration.
Coating weight too high.
Rusting.
Coating weight too low.
Final dry-off too slow.
Dry-off between stages.
Streaking.
Poor cleaning.
Poor rinsing.
Dry-off between stages.
Figure 1 1-3. A troubleshooting guide for zinc phosphatizing.
89
Shorten time.
Check racking and nozzles.
Add Jernstedt salts to rinse or
to cleaner tank.
See “Problem-Coating
weight too low.”
Increase temperature in the
fiial rinse-use air blow-off.
Better placement of nozzles.
Use fog nozzles.
Run at lower temps.
Check cleaning stage.
Keep rinses over-flowing.
Better arrangement of nozzles.
Use fog nozzles.
Run at lower temps.
Chapter 11
PROBLEM
CAUSE
REMEDY
Bath pickles metal and creates
dusty coating.
Accelerator level too high.
Reduce accelerator level by
processing aluminum or
autodraining* and adjusting the
bath.
Low coating weight.
Aluminum concentration too
high in bath.
Accelerator too low.
Concentration too low.
Total acid too high in relation
to chromium concentration.
Autodrain" and adjust.
No coating.
Total absence of accelerator
in bath.
Add accelerator.
Sludge plugging nozzles
Aluminum concentration in
bath too high.
Alkaline salts dragged into bath.
Autodrain* bath and adjust.
Add accelerator.
Add make-up chemical.
Autodrain* bath and adjust.
Increase overflow rate of rinse
following cleaner.
*Autodraining is a technique whereby the solution is simultaneously being drained and replenished
with fresh water and make-up chemical.
Figure 1 1-4 . Chromium phosphate processing solution troubleshootingguide.
90
Quality Control
PROBLEM
CAUSE
REMEDY
Red, rusty coating on mild
steel.
Temperature too low.
Increase heat to cause solution
to boil.
Dilute bath so it boils at a lowe
temperature.
Add proprietary chelating agen
at a rate of 1-6 lb./100 gal. (S2.7 kgl378.5 L).
Concentration of bath too high
for the type of steel.
Iron contamination in bath.
Red, rusty coating on high
speed or hard steels.
Concentration of bath too high for
this type of steel.
Dilute bath to boil at 255-275°F
(123.9"C) and run for 30-45
minutes.
Red, rusty coatings on iron
castings.
Concentration of bath too high for
this metal.
Dilute to boil at 255275°F
(123.9"C) run for 45-60 minutes.
No coating on high nickel
and/or chromium steels.
If these steels will coat at all, it
will generally be at high temperatures.
Increase concentration of bath
to boil at 300-320°F (148.9160°C).
No coating formed on mild
steel.
Thin, invisible oxide film on
metal surface.
Iron contamination in bath.
Iron and/or carbon contamination.
Run metal through muriatic
pickle.
Add proprietary chelating agent
Add proprietary chelating agent
or freeze out carbonated by
dropping temperature under
200°F (93°C).
White, streaky deposits on
parts when dried.
Excessively contaminated rinse.
Dump, rinse, and refill. Maintain good overflow.
Figure 11-5. A troubleshootingguide to the black oxide coating process.
91
Chapter 11
PROBLEM
CAUSE
REMEDY
Dusting-powder blowing out
of hopper.
Air pressure too high.
Adjust air regulator to lower
pressure to fluid bed.
Too much reclaim added to
virgin powder. Virgin powder
pulverized too fine by manufacturer.
Powder too fine.
No air-percolating
powder surface.
through
Insufficient air pressure.
Plugged membrane.
Obstructed membrane.
Compacted powder.
Rat holing-air blowing large
jet holes through powder
surface.
Powder level too low.
Packed or moist powder.
Obstructed membrane.
Plugged or broken membrane.
Stratification-powder separat
Powder level too high.
ing into layers of fine and
:oarse particles.
Powder too fine.
e
Follow equipment manual instructions and specifications.
Figure 11-6. Fluidized bed operation.
92
Check air supply, increase air
regulator pressure. Check air line
size to equipment.*
Check membrane for plugged
pores from dirty air supply.
Check bottom of bed for plastic,
cardboard, or other large obstructions.
Manually loosen powder and
fluidize well with clean, dry air.
Add powder until hopper is 213
full when fluidized.
Manually loosen powder and
fluidize well with clean, dry air.
Check compressed air and booth
air for high humidity.
Check bottom of bed for plastics,
cardboard, or other large obstructions.
Check membrane for plugged
pores from dirty air supply,
cracks, or holes.
Remove powder until 213 full
when fluidized.
Too much reclaim added to
virgin powder.
Virgin powder pulverized too
fine by manufacturer.
Quality Control
PROBLEM
CAUSE
REMEDY
Plugged from impact fbsionhard build-up.
Normal build-up.
Clean or replace parts.
Air pressure too high.
Turn down air settings on
pumps and guns.
Check air supply for clean, dry
air.
Check hoses.
Moisture in air supply.
Composition of powder feed
hoses.
Worn venturis and wear parts.
Powder too fine.
Powder type or formula.
Insufficient powder feed.
Powder not fluidizing.
Obstruction from contaminated
powder supply.
Kinked or flattened hoses.
Worn pump venturis.
Low air pressure.
*Follow equipment manual instructions and specifications.
Figure 11-7. Hoses and pumps-Venturi operation.
93
Replace worn parts.*
Too much reclaim added to
virgin powder
Virgin powder pulverized too
fine by manufacturer.
Some resin types tend to have
more impact fusion. Check with
your powder supplier.
See fluidized bed section.
Clean out venturis and hoses.*
Check powder supply for
contamination.
Sieve all reclaim before using.
Replace if permanently de
formed.
Avoid sharp bends. Use hose
saddles for reciprocators.
Run hoses in covered trench
across traffic aisles.
Replace worn parts.*
Check air supply.
Adjust all settings to pumps and
guns.
Chapter 11
PROBLEM
CAUSE
Poor charging-inadequate
powder build or wrap on part
High-voltage source not providing enough KV at charging
electrode or grid.
REMEDY
Poor ground.
Powder delivery (feed) is too
high.
Excessive moisture in powder
booth air.
Powder too fine.
Powder type or formula.
Powder delivery air too high.
Powder blowing by part.
*Follow equipment manual instructions and specifications.
Figure 11-8. Overall electrostatic coating operation.
94
Check high voltage source is on
Systematically check electrical
continuity from voltage source
to electrode (grid)* including
cable, resistors and fuses.
Replace missing or broken
electrode (grid).
Clean electrode (grid) insulated
by powder build or impact
fusion.
Check ground from conveyor
rail (or rub bar when used)
through hanger to part. All
contact areas must be free of
powder build, heavy grease and
other insulating material.
Turn down powder feed until
all material passing through
charging corona (field) is
adequately charged.
Moisture in humid air will tend
to dissipate humidity in the
powder spray area.
Too much reclaim added to
virgin powder.
Virgin powder pulverized too
fine by manufacturer.
Some resin types charge better
than others and some formulas
are designed for thin film
application. Check with your
powder supplier.
Turn down air setting or move
gun position farther away from
Part.
Quality Control
PROBLEM
?oor penetration-powder will
lot coat Faraday Cage areas
:holes, grooves, channels, inside
:orners, and recesses).
CAUSE
REMEDY
Powder delivery too low.
Turn up powder delivery air
setting.
Use gun barrel extension.
Check ground.
Select smaller deflector or use
suitable slotted barrel and cover.
(Consult your equipment supplier.)
Turn voltage setting down so
powder builds on part edges and
leading surfaces do not repel
powder from corner.
Turn air settings down so powder/air stream does not blow
powder out of corners.
Adjust gun position so powder
cloud has a direct path to recess
area.
Too much reclaim added to
virgin powder.
Virgin powder pulverized too
fine by manufacturer.
Turn down voltage setting.
Change gun placement away
from part.
Check ground.
Too much reclaim added to
virgin powder.
Virgin powder pulverized too
fine by manufacturer.
Adjust powder spray area
humidity. See chart for optimum
conditions.
Provide ground for all equipment*
Poor ground.
Powder spray pattern too wide.
Voltage too high.
Powder delivery velocity too
high.
Poor gun placement.
Powder too fine.
3ack charging-powder layers
ue repelled from part in spots.
Voltage too high.
Gun positioned too close to
Part.
Poor ground.
Powder too fine.
'owder picks up a random
:harge through powder or in
h i d path. Reverse charging
isually through reclaim system.
Powder booth air too dry.
Poor delivery and/or reclaim
usually through equipment
ground.
*Follow equipment manual instructions and specifications.
Vgure 11-8. Continued.
95
Chapter 11
PROBLEM
CAUSE
REMEDY
Powder feed spurting or slugging-interrupted powder feed.
Insufficient air pressure or
volume.
Check air supply. Air supply
piping to equipment is large
enough. Enough air volume
when other equipment such as
reverse air cleaning in reclaim
housing pulses, air pressure to
powder feed does not drop.*
Check powder feed hoses.
Hoses kinked, flattened, or
too long.
Hoses, pump venturis, or guns
clogged with powder.
Poor spray pattern-not a
symmetrical powder cloud
(not applicable when using
special deflectors for desired
effect).
Worn E/S gun parts.
Impact fusion build.
Delivery (feed) air too low.
Hoses, venturis, or gun blocked
with powder.
*Follow equipment manual instructions and specifications.
Figure 11-8. Continued.
96
Clean hoses, venturis, and
guns.*
Check air supply for moisture
that causes powder compaction.
Check powder free flowing
properties.
Check spray booth air humidity.
Check powder supply for
contamination. Check reclaim
sieve.
Replace worn feed tubes,
orifices, deflectors, and covers.*
Clean gun parts as needed.*
Check air supply. Increase air
for powder feed.
Clean hoses, venturis, and
guns.*
Quality Control
PROBLEM
CAUSE
REMEDY
Contamination in reclaim
powder.
Reclaim in-line sieve torn,
missing or inoperable.
Powder or dirt falling in spray
booth from conveyor or hangers.
Replace sieve or repair as
necessary.
Clean conveyor regularly (or
continuously) before entering
powder spray booth. Strip
hangers as needed.
Check cleaning and pretreatment equipment and ensure
proper part drainage before
entering spray booth.
Isolate spray booth area.
Preferably enclose in a
room with filtered, humidity
controlled air.
Contamination from parts
entering spray booth.
Contamination from plant air
circulated through spray booth.
Spray booth dusting inadequate air flow through
spray booth.
Bag or cartridge filters binding.
Final filters clogged.
Too large of open area in
spray booth housing.
Powder delivery (feed)
too high.
*Follow equipment manual instructions and specifications.
Figure 1 1-9. Collection and reclamation operation.
97
Clean or replace bags or cartridge filters.*
Check spray booth air humidity.
Check filter bags or cartridges
for powder leakage. Repair or
replace as needed.
Reduce open area.
Increased opening reduces
booth air velocity.*
Reduce the number of
spraying or the amount of
powder to each gun.*
Chapter 11
PROBLEM
CAUSE
REMEDY
Poor impact resistance/poor
flexibility.
Under cured.
Increase oven temperature (or):
Increase dwell time in oven.
Check pretreatment equipment
and chemicals.
Reduce film thickness by
adjusting application equipment.*
Check substrate with supplier.
Poor cleaning or pretreatment.
Film thickness too high.
Change in substrate thickness
or type.
Powder resin type or formula.
Poor adhesion.
Poor cleaning or pretreatment.
Change in substrate.
Under cured.
Powder resin type or formula.
Poor corrosion resistance.
Poor cleaning or pretreatment.
Under cured.
Check with powder manufac
turer.
Check pretreatment equipment
and chemicals.*
Check substrate with supplier.
Increase oven temperature (or):
Increase dwell time in oven.
Check with powder manufac
turer.
Check pretreatment equipment
and chemicals.*
Increase oven temperature (or):
Increase dwell time in oven.
~
Poor chemical resistance.
Poor pencil hardnesdpoor
abrasion resistance.
Under cured.
Increase oven temperature (or):
Increase dwell time in oven.
Powder resin type or formula.
Check with powder manufac
turer.
Under cured.
Powder resin type or formula.
Increase oven dwell time.
Check with powder manufac
turer.
*Follow equipment manual instructions and specifications.
Figure 11-70. Coating finish-cured film physical propelties.
98
Quality Control
PROBLEM
Poor surface flow-too
orange peel.
much
CAUSE
REMEDY
Film thickness too thin.
Increase film thickness by
adjusting application equipment.
Increase oven temperature (or):
Modify oven baffling to
increase heat rate.
Check with powder manufacturer.
Heat-up rate too slow.
Powder resin type or formula.
Gloss too low for high gloss
powder.
Incompatible powder contamination.
Micro-pinholing from gassing.
Powder resin type or formula.
Gloss too high for a low gloss
type powder.
Under cured.
Powder formula.
Clean application equipment
before changing powders.
Check substrate for porosity.
Check substrate for moisture.
Check powder for moisture
from reclaim or compressed
air.
Check film thickness, coating
too thick.
Check with powder manufacturer.
Increase temperature of oven
(or):
Increase dwell time in oven.
Check with powder manufacturer.
Contamination in powder.
See reclaim system: contamination, No. 1 through No. 4.
Virgin powder contaminated.
Check with powder manufacturer.
Inconsistent film thickness.
Guns positioned wrong.
Check and reposition guns so
spray pattems overlap slightly.
Adjust line speed(or):
Adjust reciprocator stroke.*
Consult your equipment
supplier.
Go through application section
check list.
Reciprocators not matched to
line speed
Air flow in booth disturbing
spray pattern.
Defective spray equipment.
*Follow equipment manual instructions and specifications.
Figure 11-1 1. Coating finish-cured films appearance.
99
Chapter 11
PROBLEM
CAUSE
REMEDY
Off color.
Improper oven exhaust.
Bake time too long.
Oven temperature too high.
Variation in film thickness.
Check exhaust vent fan(s).*
Adjust line speed.
Lower oven temperature.
See coating appearance section
inconsistent film.
Check with powder manufacturer.
Powder formulated.
See coating appearance section,
low gloss.
Pinholing and gassing through
coating surface.
Pull-away or tearing-coating
film shrinks leaving bare
substrate.
Uncharged powder.
Poor cleaning, metal
preparation or dryoff.
kFollow equipment manual instructions and specifications.
Figure 11-1 1. Continued.
100
See application section, poor
charging.
Check pretreatment equipment
dryoff oven and part drainage.
Quality Control
REMEDY
PROBLEM
CAUSE
Poor fluidizing properties
in the powder hopper.
Pressure of fluidizing air too low. Adjust (increase) pressure of
fluidizing air.
Fluidizing membrane is blocked. Clean or replace the fluidizing
membrane: see instructions of
equipment supplier.
Install an air dryer with a
Humidity of compressed air
corresponding oil-micro filter or
too high.
another suitable drying system.
Humidity of the powder too high. Check storage facilities. Powder
shall be stocked at room tem
perature in closed packing
(maximum humidity 75%).
Contact your powder supplier.
Free-flowing properties of the
powder are bad.
Blockage in venturis and hoses.
Clean or replace the venturi (see
instructions of the equipment
supplier). If necessary, reduce
pressure of powder of transport
air.
Fusing of the powder in the hoses. Clean the hose by bending and
breaking up the fused powder; if
necessary replace it.
Fusing of the powder in the hoses. Install an air dryer with a
corresponding oil micro filter or
an air dryer with a correspond
ing oil micro filter or another
suitable drying system.
Contact your powder supplier.
Bad free flowing properties of
the powder.
Fusing of the powder in the
venturi.
Figure 11-12. Output of powder insufficient to coat parts.
101
Chapter 11
PROBLEM
CAUSE
REMEDY
Blockage in the gun.
Fusing in the gun or gun outlet.
Clean the gun according to the
instructions of your equipment
supplier. When blocking occurs,
frequently check humidity of
compressed air and the freeflowing properties of the
powder.
Clean the gun according to the
instructions of equipment
supplier and determine the
reason of this contamination.
(Check powder pumps for
possible impact fusion. Impact
fusion particles which break off
in the pump could be trans
ported to the spray gun and
result in blockage.)
Blockage caused by contamination of the powder with dust
of other coarse materials.
Figure 11-12. Continued.
102
I
Quality Control
PROBLEM
CAUSE
Insufficient wrap-around.
Poor electrostatic charging of
the powder.
Adjust level of electrostatic kilo
voltage (increase). If not
possible, check equipment and
guns according to instructions
of the equipment supplier.
Check for broken electrodes on
the spray gun. If found, replace
electrodes.
Check for possible frictional
transport through powder hose.
If evident, consult powder
supplier for hose material
recommendation.
Insufficient ground contact.
Check the ground contacts using
a suitable resistance measuring
device. Correct and ensure
sufficient earth to ground
control.
Refer to problem A.
Output of powder too low.
Using an unsuitable powder type. Contact your powder supplier.
Poor penetration into corners,
flanges, slots, etc.
Output of powder too low.
Insufficient ground contact.
REMEDY
Powder cloud too wide.
Poor adherence of powder to
part, powder falls from part
easily.
Poor electrostatic charging of
the powder.
Refer to Problem A.
Check the ground contacts and
if necessary use a suitable
measuring instrument.
Narrow powder cloud. If
necessary, install a more
suitable deflector or adjust air
for cone adjustment.
Adjust level of electrostatic
kilo-voltage (increase voltage);
if not possible, check equipment
and guns according to instruc
tions of equipment supplier.
See “Insufficient Wraparound.”
Reduce powder output and/or
reduce pressure of the transport
air.
Powder output too high or the
pressure for the transport air too
high, which blows the powder
from the object.
Unsuitable particle size distribu- Contact your powder supplier.
tion of the powder or unsuitable
powder type for the objects.
Figure 11-13. Poor or insuflicent coverage
103
Chapter 11
PROBLEM
CAUSE
REMEDY
Dust, precured or other coarse
material.
Dust or other coarse parts on the
metal surface.
Dust or other coarse parts in
powder.
Precured material from original
powder which is stocked
according to the instructions.
Check pretreatment.
Check powder and locate the
cause of this contamination; if
necessary, clean up the installation and use fresh or sieved
powder.
Matting of powder surface.
Contamination with other
powder (based on other raw
materials).
Clean up the installation; if
necessary, contact your powder
supplier.
Orange-peel.
Warming up of the coated
material is too slow or too
fast.
Powder type too fast or too coarse
particle size distribution.
Moisture contamination.
Heat damage of the powder.
Check curing cycle and curing
oven; if necessary, contact your
powder supplier.
Contact your powder supplier.
Replace the powder.
Replace the powder.
Cratering.
Contamination with other powder Clean up the installation; if
necessary, contact your powder
(based on other raw materials.)
supplier.
Check pretreatment and if
Bad pretreatment with
necessary, contact pretreatment
e.g. remaining greases
supplier.
Contamination with incompatible Check the presence of incommaterials from the spraying are
patible materials; if necessary,
clean up the installation and
as e.g. silicones.
contact your powder supplier.
Pinholing.
Humidity of the powder too high. Check storage facilities. Powder
shall be stocked at room tem
perature in closed packing
(maximum humidity 75%).
Preheat objects over 320°F
Air entrapment with casting.
(160°C) and cool down before
applications (only galvanized),
or contact your powder supplier,
who can advise a special
developed powder.
Keep coating thickness below
Gas entrapment and escaping
100 microns; if necessary,
due to chemical reaction.
contact your powder supplier.
Figure 11-14. Disturbance in cured film.
104
Quality Control
1-1
Suggested Operating
Conditions
Grrlna of Ylolrtu
P u Pbund of D q A
Figure 11-15. Suggested temperature and humidity parameters for the application of organic powder coatings.
(Source: Ferro Corporation)
105
I
i -
i
POWDER COATINGS TODAY AND
TOMORROW
By Gregory J. Bocchi
Executive Director
The Powder Coating Institute
Alexandria, VA
Since its introduction in North America about 40 years ago, powder coating has become the fastest
growing finishing technology on the market. Where it competes directly with liquid finishing, powder
coating represents 15% of the finishing market and over 10% of the total industrial finishing market.
More than 3,000 commercial powder coating lines in North America are now coating hundreds of products, including automobiles, appliances, sporting goods, furniture, aluminum extrusions, and various
builder components. Powder has set a standard for durability and corrosion resistance difficult to match
with competitive finishing systems.
In recent years, increased concerns for air and water quality, along with rising energy costs, greater
consumer demand for product durability, and heightened awareness of powder and its benefits, have
caused powder coating installations to grow and expand at a rate never before seen in North America.
This growth and expansion have been accelerated by advances made in materials and equipment technology.
When powder coating was first used in North America in the 1950s. the powder system was limited in
colors, thicknesses, exterior durability, and product application. In addition, scarcity of solid resin
systems as late as the early 1970s restricted powder coating’s ability to satisfy requirements for finishing
many products.
Today, the manufacturers of powder coating materials and equipment have solved problems of the
past, and ongoing research and technology continue to break down the few remaining barriers to powder
coating. These advances have also made powder coating one of the most cost-effective finishing systems
today.
Powder Coating Materials
The most significant material breakthrough has been the development of engineered resin systems
designed to meet the diverse and specific needs of the metal finishing industry. Epoxy resins were used
almost exclusively during the early years of thermosetting powder coating and are still in broad use today.
The use of polyester resins is growing rapidly in the North American market and acrylics are a major
factor in many end-users, such as the appliance and automotive industries.
107
Powders are available with excellent resistance to corrosion, heat, impact, and abrasion. Color selection is virtually unlimited with high and low gloss, and clear finishes available. Texture selections range
from smooth surfaces to a wrinkled or matte finish. Film thickness can also be varied to suit the requirements of specific applications.
~
Development of resin systems resulted in an epoxy-polyester hybrid, which provides thin-layer, lowcuring powder coating. Advances in polyester and acrylic resins improved exterior durability of these
systems. Specific advances in resin technology include:
~
~
Thin-layer powder coatings based on epoxy-polyester hybrids provide applications in the range of 1
to 1.2 mils for colors with good hiding power. These thin films are currently suitable for indoor
applications only. Very thin films, which may require special powder grinds, can be as low as 0.5
mils.
Low-temperature powder coatings. Powder coatings with high reactivity have been developed to
cure at temperatures as low as 250°F (121°C). Such low-curing powders enable higher line speeds,
increasing production capacity without sacrificing exterior durability. They also increase the number
of substrates that can be powder coated, such as some plastics and wood products.
~
~
~
~~
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Texture powder coatings. These coatings now range from a fine texture with low gloss and a high
resistance to abrasion and scratches, to a rough texture useful for hiding the uneven surface of some
substrates. These texture coatings have undergone major improvements compared to their counterparts of several years ago.
-~
Low-gloss powder coatings. It is now possible to reduce gloss values without diminishing the
flexibility, mechanical properties, or appearance of powder coatings. Gloss values can be lowered to
1% or less in pure epoxies. The lowest gloss in weather-resistant polyester systems is about 5%.
Metallic powder coatings are currently available in an array of colors. Many of these metallic
systems are suitable for outdoor application. For excellent exterior durability, a clear powder top coat
is often applied over the metallic base. Efforts have been focused on developing perfect matches for
standard anodizing colors to meet the needs of the aluminum extrusion market. Another recent
development is the replacement of metal flakes with non-ferrous substances like mica.
Clear powder coatings have undergone significant improvements in the past several years with regard
to flow, clarity, and weather resistance. Based on polyester and acrylic resins, these clear powders set
quality standards in automotive wheels, plumbing fixtures, furniture, and hardware.
High weatherability powder coatings. Dramatic advances have been made in developing polyester
and acrylic resin systems with excellent long-term weatherability to meet the extended warranties
offered by manufacturers. Also under development are fluorocarbon-based powders, which will
match or exceed the weatherability of liquid fluorocarbons, with applied costs advantageous to
powder.
Powder coating has also become a practical finish for products that generate significant heat levels,
such as commercial lighting fixtures, and as a primer for grill tops, where it serves as a base for a liquid
top coat.
108
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Powder manufacturers continue to perfect resin and curing agent designs. Current research efforts are
focused on developing and improving lower-cost, low-curing powders to help expand powder coating
application to new substrates. Work continues in developing powders that are more durable with high
weatherability for greater use outdoors, exhibiting higher resistance to chalking or fading in sunlight.
Powder Coating Equipment
Improvements in powder coating materials have brought advancements in application and recovery
equipment technology. They are aimed at reducing the cost of powder coating systems, making the
powder coating operation more efficient, and expanding to new production requirements and part configurations. Overall material efficiency of a powder coating system commonly exceeds 95%. Equipment
engineers have made considerable progress in improving first-pass transfer efficiency and in better part
coverage to eliminate manual touch-up from automated systems. Improved spray booth and powder
delivery system designs have dramatically reduced the time and labor required for color changes. The
range of equipment available today is virtually unlimited, from the single gun, manual unit to multiple
gun, fully automated or robotic systems with sophisticated process controls and enormous capacity.
Other advances in application include:
In-mold powder coating. An in-mold powder coating process has been developed in which powder is
sprayed onto a heated mold cavity; the powder then bonds to the molding compound when the
molding cycle is performed. In-mold powder coating can be applied as a primer for a liquid top coat
or as a finish coat to reinforced plastics. It is also used on automotive body panels and bathtub
enclosures.
Blank coating. Powder coating of pre-cut metal blanks, which are then post-formed prior to final
assembly, remains a strong growth area for powder. It offers high transfer efficiency, uniform film
thickness, and a compact straight-line finishing operation that increases the level of automation.
Coil coating. Powder coatings for coiled steel and aluminum continues to be developed, with several
coil coating lines currently in operation. Wider use of powder as a coil coating will prosper as fastcuring powders, providing durable, bright, and flexible coatings, are refined and improved.
Advancements in microprocessor, robot, and infrared curing technology speed powder coating operations. These methods will continue to be adapted for more coating operations to increase their production
capacities.
Comparative Economics
Today’s environmental concerns are a major economic factor in the selection or operation of a finishing system. The environmental advantages of powder coating-no VOC problems and essentially no
waste-an
mean substantial savings in finishing costs.
As energy costs continue to rise, other advantages of powder coating become even more important.
Without the need for solvent recovery, complex filtering systems are not required, and less air has to be
moved, heated, or cooled, which can be a significant cost saving.
109
As the technology of powder coating has developed, efficiency of the process has improved. Powder is
ever more competitive with liquids, delivering quality finishes that meet the requirements of a wide range
of product applications.
~
In a study of a model coating line by the Powder Coating Institute (PCI), material costs of powder were
slightly higher than a high-solids polyester finish. Yet, the bottom-line operating costs of powder -once
costs of labor, maintenance, energy, clean-up and waste disposal are factored in -are significantly lower
than the operating costs for other systems, by about 15% for high-solids polyester, and over 40% for
conventional solvent and water-borne systems.
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Its effect on workers is one cost-cutting factor that is difficult to measure. There is minimum operator
training and supervision for a powder line. Employees prefer to work with dry powder rather than wet
solvent-based paints because of powder’s lack of fumes, reduced housekeeping problems, and minimum
contamination of clothing.
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Ongoing technological advancements in materials, equipment, and application techniques ensure that
powder coatings will occupy an ever-increasing share of the finishing market.
~~
110
INDEX
A
D-E
Abrasive blasting, 27,28
Acid cleaners, 29.32.36
Acrylics, 11
Airblast, 26
Airflow, 68
Airless blast, 26
Air plenum, 48
Alkaline cleaners, 29,31,35
Aluminum oxide, 28
Applications, 2
Deionized water rinse, 31
Economic advantages, 18
Electrocleaning, 29,32
Electrostatic charging medium, 42
Electrostatic coating operation, 94
Electrostatic fluidized bed
applications, 44
design, 46
equipment characteristics, 45
powder characteristics, 44
safety considerations, 46
Electrostatic Powder Spray Coating
ease of application, 49
economic advantage, 48
equipment, 49
Electrostatic spray, 39
Electrostatic spray guns, 50
Energy savings, 17
Environmental implications, 2
EPOXY
functional, 7
polyester hybrids, 8
resin-based, 7
thin-film, 7
Excess heat, 77
B
Back ionization, 53
Batch booth, 56
Bed density, 41
Belt booth, 59
Blocked diffusion, 41
Burn-off ovens, 82
C
Calrods, 74
Capital costs, 18
Cartridge mounting arrangement, 63
Centrifugal wheel, See: airless blast
Chain-on-edge booth, 59
Chemical strippers, 82
Chromium phosphate, 34,W
Cleaning, 25
Cleanliness, 68
Cold-rolled steel, 34
Cold water rinse, 31
Collector inlet design, 63
Combustion chamber, 74
Convection heating, 74
Corncob, 28
Cost savings, 17
Cross hatch test, 82
Cryogenic stripping, 82
F
Faraday Cage, 51,53
Feeders, 49
Finish-cured films, 99
Fluidization, 41
Fluidized beds, 39,92
Functional Epoxy, 7
G
Gages, 68
Garnet, 27
Glycidyl acrylics, 11,12
111
Index
Gravity-assisted booths, 56
Gun spitting, 77
OSHA, 78
OSHA Section 1910.107,71
H
P-Q
Handling powder containers, 78
Hand-wiping, 29,31,32
Hangers, 82
Health hazards to operators, 70
Heating components, 73
Heating operations, 73
Heavy mineral sands, 27
High moisture content, 77
Horizontal coating booth, 58
Hoses and pumps, 93
Hot water rinse, 31
Housekeeping, 86
Humidity, 77
Hygiene, 78
Peach pits, 28
Pecan shells, 28
Polyester
carboxyl, 10
new, 11
urethane, 9
Polyethylene, 6
Polypropylene, 6
Polyvinyl, 6
Postheating, 73
Powder coating markets, 2
Powder delivery hoses, 50
Power spray method, 29,30,31
Preheating, 73
Pretreatment system, 25
Quick-color change booth, 56'
I-L
R
Immersion method, 31,32
Infrared radiation, 74
Intercoat adhesion, 82
Internal charging guns, 51
Iron phosphate, 36
Labor savings, 17
Lower explosion limit, 75
Rebake, 82
Recoat, 81
Recovery of oversprayed material, 55
Resistance emitters, 74
Retractable polyethylene sheeting, 55
Reverse-jet cleaning mechanism, 63
Rigid booth, 58
M-N
S
Magnetite, 27
Material Safety Data Sheet, 70,78
Mechanical blasting, 28
Migrating powder, 67
Modified silicone, 13
Neutral cleaners, 30,33
NFPA Bulletin 33,71
NFPA Industrial Fire Hazards Handbook, 71
Novaculite, 27
Nozzle blasting, 26
Nylon, 6
Sandblasting, 26
Self-contained booth, 59
Shot blasting, 82
Side draft spray booth, 60
Sieving devices, 49
Silica sands, 27
Silicon carbide, 28
Silicone, 13
Skin irritation, 19
Skin reactions, 78
SPC,See: statistical process control
Spray booth, 55
Statistical process control, 85
Staurolite, 27
0
Olivine rutile, 27
Open collector cartridge filtration, 61
112
Index
Storage, 78
Stratification,41
Stripping, 82
System Components
operation of, 71
T
Thermoplastic, 5 , 6
Thermosets, 5
Thin-film epoxy, 7
Touch-up, 81
Tribolectric charging guns,
Troubleshooting
black oxide, 93
chromium phosphate, 92
iron phosphatizing, 90
zinc phosphatizing, 91
,
I
-~
-
i
u-v
c
a
Uneven fluidizing distribution, 43
Variables, 85
Venturi, 47
VOCs, See: Volatile organic compounds
Volatile organic compounds, 1
w-z
Walnut shells, 28
Wrap, 53
Zinc phosphate, 34,38
Zinc phosphatizing, 89
Zinc phosphate processing solution, 3 1
Zircon, 27
i
113
t
t
114
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i
I
i
i
f